Well, I'd avoid "gay marriage", first of all... "Same-sex marriage" is more precise. I think it was Greg Koukl who pointed out that marriage has never had any restrictions on the sexual orientation of those involved...we haven't had "heterosexual marriage" up until now, we've had opposite sex marriage.
Remember that the people you are conversing with are individuals. "They", "them", "those", are words to avoid when referring to people groups as it serves to set tribal lines from the get-go and people are very tribal beings. I have some gay friends and at least one relative, and each of them is a person with personal opinions and nuances... they don't want to be lumped into a group and given supposed motivations that they don't feel like they have. (lots of 'theys' there, huh?) A statement like "Gays just want to have the rest of us X Y and Z" is easy to refute for a gay person who doesn't have (or recognize) those motivations. If you'd be offended at being lumped into "Christians hold 'God hates fags' signs" then you should understand that the LGBT community is less a community and more a collection of very individualistic individuals (who just happen to have a few issues they hold in common).
Promote brotherly and Christian love. One old classmate of mine got very offended at my opposition to same-sex marriage, and when he challenged my "hateful" statements, I was able to ask him if I have ever treated him as less than human or even less than friendly... I pointed out that I am looking out for what I see as his best interests because I love him: if I didn't care for him I wouldn't care what he did with himself, even if I still recognized that we disagree strongly on what his "best" is. We definitely don't see eye to eye on a lot of things, but we can talk civilly and even have fun with each other at times. I guess what I am trying to say is that he is less offended at my opposition to his decisions because I've shown him I care for him than if I let myself just be disgusted by his decisions.
One good way of doing that is to show interest in his life outside of his gayness... I'll look out for posts that are non-gay related and like and comment on those, so that he knows I care for him, not just about his lifestyle. How's work? I'm jealous of the nice weather down there. Your kitten is adorable. and so forth.
Next, remember that there is a difference between "Christian" and "American". With Nathan J Norman's trip to Vietnam on our minds, we should see that "Christian" is going to mean something very different than "American" or even "Western" very soon (historically speaking). East Asia is the future center of the Christian faith, if trends continue. Our nation may have been founded on Christian principles, but we aren't there anymore. Keep that in mind when you get angry or offended at court decisions, and realize that we are told not to expect adherence to the Law by non-believers. (I can't remember the reference right now) I know I tend toward cynicism, but what do we really expect from the God-less.
Speaking of which, not all LGBT/LGBT supporters are non-believers. Now, I tend to think they are mistaken in this area, but there are other Christians out there who have no problem with "little white lies" and gluttony and reviling and so forth as well.
Which leads me into my last (unless my mind catches on something else) point: We shouldn't hold homosexual behavior as somehow worse than our own sins, even while we recognize that repentance is important. I often make it a point of stating that the sin of homosexual activity is no more important than any other sin, including my own, and that it is the topic brought to the fore by people other than myself, and if we were having a discussion on adultery or idolatry, I'd be just as against those as same-sex marriage. (and probably more with idolatry)