I agree with the people saying that it wouldn't really be 'beastiality' anymore for one of the Turtles to be with April. Honestly, I'd say if one of them were to be with an actual turtle would be beastiality more than them having a relationship with April. Yes, they may be a different species from April, but as they're all sentient, I'd say it's up to them - or would be if they were real and all. And it would be a pretty interesting plot point to see how the world would react in-story.
Not that I'm expecting the Nick show to take advantage of this idea.
