I suppose just that for much/all (depending on version) of their lives human women were just always their only/main point of reference when it comes to fellow intelligent, bipedal, sentient beings who happen to be female. Esp if they pretty much feel the same as us on the inside.
And we're the weird ones who are relatively bare skinned compared to most other mammals and apes, with human women tending to remove even more of it. I could see why they might be willing to look past differences. Even if humans are too hung up on differences, even among ourselves, to return the lack of judgement.
Though I'd be curious to know a little backstory on Raph's attraction to these furry women.
Maybe along the lines that some people end up finding someone that in some way is similar to a parent? They're furry like Splinter? lol Makes sense that he'd say, "For a human" about April. Humans just don't seem to be much his type, in most cases.