Let me first say that I am extremely empathetic to the situation in which gay and lesbians find themselves. I wish I could give an answer that made me feel "fair" about it, but unfortunately, life is not fair. Each of us have temptations and vices we must deal with and no one escapes the fact that they must deal with sinful desires. Having said that, my answer is yes, it would still be a sin. Honestly, whether or not someone is allowed to legally marry is not the issue (in my opinion). It is whether it is a marriage recognized and blessed by God. Let me add that many heterosexual marriages are probably no more spiritual or "right."
I believe that if someone is homosexual, they have been called to be celibate, unless their struggle ends in them changing preferences (which is extremely rare). It is a high calling and a difficult one. However, I believe it is the right path to choose.
I want to elaborate a little on your comment about "right-wingers" so you know a little more about where I come from. I believe each person must choose their path. If someone asks me if homosexual behavior is a sin, I will tell them I believe it is. However, I do not believe our government should try to legislate their behavior. I believe in one's freedom to choose to live as they desire. I do not force my beliefs on them. I would not treat them any different than straight people, for we are all sinners and no sinner is better than any other. Just because I was not born (or did not develop into) a homosexual does not mean I'm better, worse or anything else. It just means that's not the sin I'm tempted with.