Okay I just made my account today asking one question. Why is my male guppy trying to stick his wo-ha in another male platy's bum? I have a 10 gallon tank with all males. 2 guppies and 2 platies.
It's perfectly natural and normal; male livebearers are sex mad, that's all. They'll mate with anything; they don't really care what sex or species the other fish is!