Yeah, I can relate to this. I've been arguing for same-sex marriage on different sites, but particularly on Facebook. Damn, you get some stubborn resistance! Most people I can refute with facts (having a reasonably high IQ helps). Some people I actually enjoy discussing issues with, even if we disagree.
And then you get some people who just DESERVE to be smacked down.
I'll admit that while same-sex marriage is a hot-button topic, it's hardly the most important fight that the LGBTQ community has on its hands. Discrimination exists in many forms, but every bit of alleviating ignorance and fear helps. It's been a hard slog thus far, and it will continue to be for some time. At least we've made an awful lot of progress, especially in the last 10 years.
Just remember to be persistent, state your position clearly without getting too sidetracked and be prepared to back up what you say with hard facts. And be civil: no matter how much you want to yell at someone to "shut the f*ck up!", they're not going to turn around and say "okay, you're right, I'll shut up now".
Don't be afraid to stand up for what you believe in. It's the only reason we've made as much progress as we have. There will always be people who appreciate your efforts and persistence.
_________________
It is easy to go down into Hell;
Night and day, the gates of dark Death stand wide;
But to climb back again, to retrace one's steps to the upper air –
There's the rub, the task.
– Virgil, The Aeneid (Book VI)