In the song Froggy Went A-Courtin', isn't it kind of weird that the frog marries Miss Mouse? And why does Kermit date a pig? What's with frogs dating outside of their species? Frogs aren't attractive... that may be part of it. Another reason may be that it's very difficult to tell what gender a frog is, which is why frogs gravitate toward a Miss Mouse or a Miss Piggy... no confusion there. And maybe since frogs look so similar, inter-species dating is the best way for them to ensure they're not dating their sisters.