--Disclaimer---
This story is entirely fictional and doesn't/shouldn't represent any real life situations. It is strictly for entertainment purposes.

It was an unusually wet few weeks in the Mushroom Kingdom. Torrential downpours kept most confined to their homes. Both residents and monsters across the land were left to simply hibernate indoors and wait it out. That is all except for Bowser and his koopa troop. The rain provided them the rare opportunity to let the waters into the castle and heat their baths warmed by the pits of boiling lava. 
Usually the koopas found their own places around the Kingdom to clean themselves. There were plenty of waterways, so the facilities in Bowser's castle were usually bone dry. However, this was far from the case now. With shells loosened and discarded, nearly the whole castle had come together to relax in the large bath hall. Similar to traditions of old, the hall was one giant bath shared by everyone. The koopa troopas weren't shy about being seen without their shells either. Most were just happy to enjoy the hot water regardless of having their slits exposed. 

On the far end of the bathing hall was Bowser's private bath. Walled off and out of sight, he and Junior sat back on either side of a star-shaped pool of bubbling water. Unlike their smaller koopa counterparts, the father and son duo did their best to hide their sensitive areas from the other. They were both in need of the bath, and Bowser didn't want things to escalate. Despite his best efforts though, Junior’s teenage desires would soon get the better of him. 

“This feels so good, dad!” Junior said, his youthful voice causing Bowser to perk up. Despite being nearly 13, Junior still sounded very young. His higher pitched voice seemed to pierce the silence any time he spoke. 

“Sure does son, you were maybe only 4 the last time we could open the castle's baths. Feels great to finally share it with you.” He paused for a moment, then realizing how it sounded started stuttering. “No, no, I mean like it’s nice you think so, not that I…”

“Pffft, hahaha,” Junior laughed, kicking his legs causing water to splash around the tub. “Geez Dad, I didn't think you'd be shy around me at this point.” 
Despite knowing they needed the bath, Junior was still a horny young koopa and knew it wouldn't take much to get his father worked up (if he wasn't already). The two of them had become intimate about a year earlier and spent nearly every night in bed together. It was a wonderful arrangement for both of them physically and as family. Their bond had never been stronger once they became lovers. Given his age Bowser's libedo wasn't as strong as Junior's, but that isn't to say he didn't have needs. Getting the castle's bath hall ready for several hundred koopas kept them apart for the past week. Both of them were so pent up that even seeing each other without their shell made them tense and dripping with desire. Seeing how flustered Bowser was, Junior decided it was time enough for a little fun. 

“Hrmph,” Bowser grumbled, folding his arms to his chest. “I just wanted you to appreciate the work it took to make this happen. You're lucky you're cute.” 
Junior smiled at his father's obvious comment. He could tell Bowser was trying his best not to get aroused. This opportunity was perfect for him to tease the koopa king until he got what he wanted, and they both needed.
“Oh I'm very grateful Dad,” Junior said, eyelids lowered as he swam across the tub and slid next to him. Placing one hand on Bowser's inner thigh, Junior caressed him gently. 
“Perhaps I can show you just how happy I really am.”
Bowser closed his eyes and snorted a puff of smoke from his nostrils. Gritting his teeth he knew exactly where this was headed. As much as he desperately needed release, he was still hesitant. Letting Junior take such a dominant first step was a punch to his pride. While he had bottomed for him before, this was very different. He'd be letting Junior take complete control.
“Come on Dad,” Junior whined, sliding his claws to the edge of Bowser's slit. “I know you wanna do it. I really need your cock.”

“Gah!” Bowser gasped, his eyes shot open, and he turned to Junior who had poked the tip of his claw inside him. Seeing Junior's seductive face, he relented and sighed. “Ok son, you win.”
Arching his back, spreading his legs, and reaching out with both arms, he supported himself on the bath's edge for his son to get started. Junior smiled and pulled back his hand, licking Bowser's slit juice from his claw. Crawling over the big koopa's leg, Junior stood in the bath with his own slit just barely covered by the steamy water. Bowser had pushed himself up on the edge, leaving only his lower legs submerged. Licking his lips, Junior got a light waft of Bowser's musk from being so close. 

“You're so freaking hot, dad.” Junior mumbled, as he pressed his muzzle to Bowser's slit, nudging it. “I…I…aaaaallll,” Not wasting any longer, Junior eagerly lapped his tongue across his father's slit. He heard Bowser grunting, trying to hold in a louder moan. Placing one hand on each of the big koopa's thighs, Junior gleefully licked up and down across Bowser's slit. It wasn’t long before the tip of his cock poked out, which Junior was quick to suck on as it grew larger and larger. Just the sight of Bowser's thick, red, tapered cock got Junior hard himself. The young koopa was an impressive 7 inches of pure reptilian majesty. It stood proudly out of the water, twitching and leaking drops of precum. As wonderfully big as Junior was for his age, he was nothing compared to the king. Sitting back in awe, Junior always took a few minutes to admire his father's manhood. The monstrous 18 inch cock towered up to his head, throbbed over and over, and drenching its length in warm slick precum. 

Bowser snorted again, this time trails of fire escaping with his breath. He watched closely as his son ran his sharp fingers up and down his manhood. Junior's gentle touch sent shivers of pleasure through his loins. There was something magical about his touch that was better than any other partner he'd had in the past. Nothing was better or got him more horny than what his own son could do. 
“Ahhhh, that's my boy.” 

Junior smiled, and stepped out of the water. Climbing on Bowser's legs, he knelt on them and pressed their cocks together. Humping slowly, he lubricated his boyhood with his father's precum while spreading around his own. 
“Hehe, You're really leaking a lot today. Does my cock feel that good against yours?” 

“Ahhh, yeah.” Bowser replied, wrapping his hands around Junior's butt. He rubbed in circles, occasionally running two claws up and down the young koopa's short tail. 
“You're such a good boy. Daddy loves his prince's big cock.” 

“Hehe, not as big as yours though.” 

“You're already bigger than you were a month ago, you know. I bet you'll be up to a…AH!...a foot by the fall.”

“Umph, r…really? I haven't noticed,” Junior sheepishly admitted.

“I started noticing your body getting bigger a few…ah…weeks ago actually. You'll be as big as me before you know it.”

“Well then,” Junior said, scooping a handful of precum up and rubbing it just below Bowser's slit. “I guess we gotta keep you up to date with my changes.” Standing up on Bowser's lap, Junior whispered seductively into his father's ear, “Lay back dad, I'm gonna fuck you now.” 

Bowser's cock throbbed, shooting a long stream of precum into Junior's chin. The younger koopa wasn't even phased by it. Gulping as his son slid off of him, he looked down as the teen eagerly waited for his compliance. Seeing Junior take control was so unbelievably HOT! Bowser was simultaneously proud and afraid as his son's stiff boyhood twitched before him. Laying back as he was told, the older koopa spread his legs wide, exposing his tailhole. For some reason he was nervous. The dark lustful hunger in Junior's eyes was causing his heart to race. 
“R…ready,” Bowser stuttered. 
Wasting no time, Junior angled his cock and began prodding at Bowser's tailhole. The younger koopa grabbed his father's hips, and in one swift motion plunged all 7 inches into him at once. 
“Argh!” Bowser grunted, a blast of fire escaping his mouth. He hadn’t expected Junior to start so roughly. Despite being well coated in his natural lubrication, it was still a lot to take in all at once. It was also immediately clear just how much Junior's cock had grown since the last time he was a bottom. His son was hitting much deeper now, and was a bit thicker at the base. Bowser couldn't believe how quickly Junior had begun to mature. Puberty was hitting the young koopa like a truck, and his cock was getting most of that growth.

“Ah, yeah!” Junior moaned, beginning to pump his hips. The wet sounds of his boyhood violating his father's body echoed through the room. He watched as Bowser's monster cock throbbed wildly as he picked up the pace. The koopa king's muscular form wrapped tightly around his younger cock. This was a feeling Junior had grown accustomed to and loved the warmth he felt while penetrating his father. 
“Yeah take it dad!” He shouted, grinding harder and faster in and out. “You love getting fucked by your son don't you?”

“Y…yes!” Bowser moaned. Junior's dominance was driving him wild. “Don’t stop, son! Breed me!” 

PLAP PLAP PLAP PLAP PLAP PLAP 

Their breathing was heavy and their moans loud. Having forced themselves to avoid intimacy for a whole week had driven both of them into an intense heat. The more Junior pounded him the better it felt. Bowser was completely entranced by his son’s overwhelming sexual energy.

PLAP PLAP PLAP PLAP PLAP PLAP 

Feeling a climax building, Bowser wrapped his legs around Junior's back, forcing the young koopa to keep his cock buried inside. 
“Fuck son, I'm cumming!” 

“M…me too, Daaaaaaaad!”
Just as Junior began pumping Bowser's tailhole full of his youthful seed, the koopa king too reached his climax. Both reptiles’ cocks throbbed uncontrollably, blasting ropes of cum from their bodies. Bowser letting out a long fiery moan, could feel the countless streams filling his body. At the same time his monstrous cock sprayed a massive load into the air, which showered the two of them in ropes of thick musky love juices. The two koopas moaned as their pent up desires were released all at once. Every passing glance and dirty thought from the week was unleashed in their mutual orgasm. Finally letting his legs relax, Bowser let his son pull out as they fell from the peak of pleasure.

SHLOOP

Having been ravaged by Junior's cock, Bowser's tailhole was loosened to the point of oozing his son's cum back onto the stone floor. Breathing heavily, the pair of koopas looked into each other’s eyes and smiled.
“I…huff huff. I love you son! P…please…m…more.”

“Pft…you perverted old koot.” Junior chuckled, laughing at him. Stepping forward again, he slowly slid his 7 inches into Bowser's leaking tailhole one more time. “You're lucky you're cute.” He said, returning his father's earlier compliment. He was happy to hear his father begging for more. Junior knew immediately that one time wouldn't be enough anyway. They were both still rock hard, and their bodies ached for one another. 

SSSSSSSSHLICK

“Oh, he'll yes!” Bowser snorted,yet another burst of fire passing his nostrils. “Give me all you got!”

PLAP PLAP PLAP PLAP PLAP PLAP PLAP

The two of them continued having sex over and over and over. Each reaching climax within a minute of the other every time. The floor of their private bath quickly became painted the white of their combined cum. It filled the room with a potent aromatic aphrodisiac, which only further fueled their passions. They must have each cum at least a dozen times by the time they finally became too tired to continue. Slumping back into the steamy bath, Junior sat just in front of Bowser as they slowly became flaccid in the bubbly water. Leaning back on his father's chest, Junior smiled knowing he had filled the koopa king with every last drop of his princely cum. 

“Thanks again for the bath, dad.” Junior said, just as his boyhood slipped back into his slit. The rush of warm water cleaning the remaining cum from his body. 

“You're welcome, son. Thanks for breeding my tailhole into oblivion. Do you…wanna sleep with me tonight?”

“Of course!” Junior said, turning his head. “D…do you think I can cum inside you one more time before we go to bed tonight?”

Bowser leaned down and pressed his lips to Junior's. His tongue slipping inside, they shared a short but passionate kiss. 
“Mwah. Oh I'm counting on it, my sexy little koopa prince. My tailhole is all yours.”