Raph knew why he gave Donnie a hard time about April. It wasn't because his brother didn't have a chance with her. It was because he was scared that Donnie did.

Raph wasn't as obvious about his crush on the human, but that didn't mean his feelings weren't as strong.

It took a long time for him to realise why he was okay with Casey's relationship with her.

Casey was human. It made sense for April to like him... if April liked Donnie, he couldn't pretend that he never had a shot at her just because he was a mutant.