But we're not discussing whether or not to throw two gays in jail just because of their desire to be "married". Rather, we are deciding whether or not the government should extend certain legal benefits to them for doing so---whether it should promote the practice. When it comes to promotion, the government has signficantly more latitude, constitutionally speaking.
I liken the legal benefits of marriage to be akin to the ability to take tax deductions for donations to qualifying charitable groups. Obviously the incentives are much larger for marriage, but the promotion concept is the same. Many of us choose to make donations (or make them larger) precisely because of this practice. And yet why should the government bother with this? Some libertarians would certainly argue it shouldn't. But I say that the government does have an interest in fostering and encouraging certain social activities or institutions that benefit society, and I consider heterosexual marriage specifically to be one such institution. Unfortunately we as a society have done many other things to cheapen that institution (no-fault divorce for example), but that is no excuse to visit wrongs with wrongs.
So back to your example. You ask me whether I would prohibit drug addicts or alcoholics from getting married. Well, no, because that would be a prohibition, and to enforce it I would have to prove that this person or that is an alcoholic or drug addict. But when two men walk into a courtroom and ask to be married, there's really nothing to prove, is there? It's simply impossible for them to fill out the paperwork because they can't complete the line that says, "Wife's name."
And again, I have never denied that some homosexual couples can do as well as some heterosexual couples at raising children. In fact I have specifically said so many times. But this is not about allowing marriage only for people who would be good parents. That's simply an impossible standard to enforce. This is simply about the government using its powers of promotion in a targeted fashion so that it promotes the ideal family structure. The government is making the simple acknowledgement that the best heterosexual parents will always be better than the best homosexual parents (at parenting), because the provide certain developmental needs that homosexual couples simply can never provide.