Christians aren't seeking to outlaw adultery.
People aren't seeking to outlaw homosexuality either. Just same sex marriage.
It's be ridiculous to try and outlaw adultery or homosexuality. Because even though they are both sins, we can't control other people's actions. We can tell them that their actions are sinful, but that's just about it. Trying to actually force them to stop what they're doing is wrong. It's not our place to force them to stop. That's God's place. But what we as Christian CAN do, is prevent stuff like same sex marriage from being allowed.
The government has basically put the decision of whether gay people should marry or not in our hands. And what decision you make on it is yours. But I know what I'm gonna do. I am gonna say no to letting it happen.
And just because homosexuality was mentioned with the laws in the OT doesn't mean anything. In the OT it says that idolatry, lying, murdering, and stealing are sins. That's the Law of Moses. What argument are we gonna come up with that? That we can lie, steal, murder, and worship anything we want? Umm. I don't think so. It also says in the New Testament that those are sins. Same with homosexuality. It is mentioned in the New Testament that homosexuality is a sin. Right in Romans 1. Romans 1 IS NOT about idolatry. Yes, it mentions idolatry in verse 25, but that's not what it's ALL about. Look at it closely.
Romans 1:26-27
26 For this reason God gave them over to degrading passions; for their women exchanged the natural function for that which is unnatural, 27 and in the same way also the men abandoned the natural function of the woman and burned in their desire toward one another, men with men committing indecent acts and receiving in their own persons the due penalty of their error.
Though Romans 1 isn't ALL about homosexuality either. It's mainly about the immorality of man. But homosexuality is included because it is considered sin. And immoral.