This is a weird argument to be having here, especially because it is a trans issue, whether you know it or not. Married trans people have been shut out from their legal spousal rights and even have had their marriages annulled after-the-fact by ex-husbands and greedy families after their happily-married-to-a-trans person family member died. Those arguments always make the case that we are not real men and/or women, whichever happens to be convenient. If same-sex marriage is legal, then those arguments have zero grounds.
Also, depending on the state you are from, you may be allowed to marry only men, only women, or even both, depending on where you are in your transition. Also I believe Texas was trying to enact a law that didn't let us marry anybody cause they can't figure out our "real" sex. Idk if that passed. Oh, btw, even if you do marry the sex your state allows, your marriage is always in danger of being taken away from you because of the stuff I mentioned in the first paragraph. And even if that nightmare doesn't happen to you, then soon as you cross state lines you may be still or not or maybe married depending on the state you visit or move to. Do you see the mess? It is immediately untangled by gay marriage being made universally legal.
And don't get me started on the cost of not being able to be legally married or the spousal rights, which are SIGNIFICANT, that we deny a very large portion of society because of how they were born. That is why this IS is a civil rights issue. Also, what actual doubt is there about the nature vs. nurture origins of homosexuality, and what does that even matter when, however somebody turns out gay, we have proven a bajillion times over there is no changing it?