How can Christians reconcile the request to prohibit same-sex marriage and the lawfulness of homosexuality itself? Assuming one believes that the bible is the word of god, believes that the bible says homossexuality is wrong (or commonly quoted, an abomination), and because of that gays should not marry, how come one says same-sex marriage should be unlawful and homosexuality itself should not? In other words, if one wants to prohibit same-gender marriages because homosexuality is an abomination, shouldn't one be proposing to make it unlawful just to be gay or act upon homosexuality such as build a household or simply have sex with someone of the same gender? 

Edit: I would like to clarify the point of my question a little better. I am not really interested in if Gay Marriage should be outlawed or not (I personally believe it should not, but that's besides the point). Of course I am also not suggesting the viability of attempting to outlaw homosexuality itself, which is absurd in our society.
My point is that I see people DEFENDING certain rights of homosexuals' such as the right to have homosexual relationships, public display of affection, the right to constitute a family, adopt, etc, and at the same time say their marriage should be banned because homosexuality is a sin/abomination. That is the disconnect I am trying to understand.
I don't know anyone who says that he believes homosexual marriage is bad but homosexual sex in general is okay. The nature of the current legal debate is that homosexuals are demanding a change in the laws regarding marriage, which some who believe homosexuality to be a sin are opposing. If I understand you correctly, you are saying that you see an inconsistency if someone opposes gay marriage but does not at the same time campaign to make gay sex in general illegal.

But that's the nature of political debate, isn't it? You can't deal with every possible issue at the same time. It's not inconsistent to fight for or against one particular law without simultaneously campaigning for or against every other law that exists on any related topic, plus every law that you can imagine that might be passed. You can't deal with everything at once. You can only fight so many battles at one time.

Suppose during the 1950s someone campaigned against racially segregated schools. Would you say that he is "inconsistent" because he does not also campaign against job discrimination? Maybe he doesn't have the time or resources to conduct both campaigns. You have to pick your battles.

There's also the whole big question of the relationship between law, morality, and religion. Should Christians work to make everything that the Bible calls a sin made illegal? Or, even assuming that that was politically possible, would that be an inappropriate role for government?

I think a case could be made that the government should have laws defining and regulating marriage, because these are relevant to taxation, inheritance, and other things that are routinely considered legitimate functions of government. And if the government must define it, then it cannot escape the question of whether a gay couple can or cannot be considered married. But what gay people do in their own homes does not necessarily impinge on legitimate functions of government. (Personally, I'm not really convinced by this argument, but I think it's reasonable.)