I agree with what a guy said about it in an interview I read; the main reason is probably that most people were still kids when it last happened. When you think about it, most fans of these shows are going to be at the age where they think all romance is gross, but they aren't sexually mature enough to find interspecies romance uniquely objectionable. Also because of the young age demographic, there's going to be a limit to how far they will go. It's not like they can show Donatello having sex with April.
Then again...