Hmm, I guess we could post articles about it being documented in various other species but there are two issues there. The first is I don't think anyone really opposes the idea that it happens in nature anyway, and the second is that there's nothing to say we have to do certain things just because animals do. For example throwing faeces at people is natural, but its generally considered bad form all the same. Just because something is natural doesnt make it right/wrong.
In the same vein we could try to show that some humans are born gay, but
nobody has ever been able to prove that one way or the other. And once again if they did, it wouldnt make any odds. Homosexuals still exist, either way.
Being pro homosexual presumably means either being homosexual yourself or at least supporting their rights to have relationships, marriage etc? This is coming up more and more in the UK right now, our government wants to legalise gay marriage by the next general election (so by about 2015).
Public opinion polls show that general support has been increasing over time. Again though, it doesnt matter. Its either right or it isnt, and just because most people support something doesnt make it right/wrong.
The biggest piece of evidence for me is that there simply isnt a reason to try to ban or suppress homosexuality. That reason is enough that the attempts to do so and the inequalities in society deserve to be fought.