Aineo,
Post hoc, ergo propter hoc. That's the Latin for the fallacy in question -- I just remembered it. And as far as I can see, your source is a textbook case. He's concluding causality from sequentiality -- i.e., B happened after A, therefore A caused B. This is <I>not</I> a valid line of reasoning, even if A and B are indisputable events. I don't need to study history to disprove his conclusion -- not unless there's a lot more to his argument than you've presented here.
And no, I can't categorically state that "such relationships" are monogamous. Of course not. I can't categorically state that <I>any</I> relationship is monogamous. I don't see what that has to do with anything, frankly... except that you apparently think that gay people are inherently incapable of commitment and/or monogamy, which justifies prohibiting us from marriage. Just out of curiousity, assuming that there <I>are</I> gay people out there who are both committed and monogamous, do you believe that that's fair to them? Exactly how many committed-and-monogamous people does it take before you'd agree they <I>shouldn't</I> be discriminated against for being part of a group which is statistically less faithful than the norm? (For the sake of argument, since I've yet to see any compelling evidence.)
"All major religions oppose gay marriage" is not a point I find relevant. Marriage, in the legal sense, has nothing to do with religion. "All major religions" can go right on opposing it, as far as I'm concerned, as long as legal marriage is made available to heterosexuals and homosexuals alike. To put it another way, I have no interest in convincing anyone that I'm <I>right</I> -- merely that I should have the right to be wrong. And I see restricting marriage on the basis of any <I>or</I> all religions as a violation of the First Amendment. From my perspective, that makes it the weakest of your arguments... and needless to say, I don't see any of them as being very strong. But if I've missed something here, please, bear with me and point it out.
Just remember, what you see as "explaining away and rationalizing," I see as a perfectly good rebuttal. I'm not having this argument because I <I>don't</I> believe I'm in the right, that's for sure. I am sincere, and I still don't understand how so many people end up on what I see as the obvious wrong side of this issue... an issue which I'm inclined to take quite personally, since it affects me far more than it affects any of you. That's where I'm coming from. And if I can continue to discuss this with you folks without throwing up my hands and declaiming the futility of it all, then I'll hope that you can do the same.