I'll have more to say a bit later, but for now...
Y'ever run into that particular sort of self-proclaimed Christian who is totally against gay marriage you guys, but they just happen to think it should nevertheless be legal?
In fact, they are ADAMANT it should be legal, willing to argue acrimoniously for days upon days about why it is of the utmost importance to legalize it and reject the legitimacy of any and all arguments against that?
Now and then they'll drop, as an aside, 'Well I'm against it for religious reasons, I am *totally* against it being in the church', but they seem suspiciously unwilling to talk about that in much detail. In fact, it almost seems as if their mention of their opposition to gay marriage in any way largely shows up purely so they can purchase a bit of credibility among their 'fellow Christians'.
Add in a history of out and out deceiving people about their opposition to even legal gay marriage - arguing that it should be dropped for now, and revisited in a hundred years because 'Souls are at stake! We're turning people off with this fight!', before it coming out that actually they support gay marriage being legal, full stop - and really, what you have is someone who you'd be delusional not to regard as a middle-of-the-road, all-too-common bullshitter.
I say this because I think some Christians don't know what to do when they encounter a dishonest person like this, save for try to catch them in a lie. See if they slip up. And you know what? You'll manage to catch them contradicting themselves, lying, or otherwise. But they'll always offer up an excuse, however poor of one it is. They'll oscillate between unbelievably hurt that you'd ever question their sincerity, and tremendously hostile - whatever seems like it may pick up sympathy at the time.
My advice? Don't be afraid to simply say 'Well, you're full of shit and don't deserve conversation', ignore them, and move on. Cite their lies or their inanity of their stance - which will be obvious to anyone who's actually interested in sincere discussion - and then move on to better conversations, of which there are an endless amount available online, since the internet is the place where nobody stops talking.
Just a bit of friendly advice from a person experienced with these kinds of people.