Joe Carter at the Evangelical Outpost has a list of studies supporting his contention that homosexual men are less interested in long-term monogamous relationships than heterosexual men. These studies are consistent with statistics I've seen before, but I don't think they get to the heart of the gay marriage issue, which is what Joe is addressing.
Let's say that Joe is right and as a group homosexual men are uninterested in monogamous relationships. What about those who are? Those advocating gay marriage are arguing that those homosexual couples who do want to marry want a monogamous relationship. This may or may not be the case, and Joe points to some statistics that suggest that what they want are open marriages. However, this is not easily proven, and those advocating gay marriage will argue strenuously that while some may want that, that is not what they themselves are asking for when they argue for gay marriage. They will continue to argue that once the ideal of gay marriage is in place, monogamous norms will become more common among homosexual couples. Ideally, marriage licenses could be denied to anyone who was asking for an open marriage, regardless of sexual orientation, but there is no way to determine that beforehand, and no way to enforce it afterwards. (This might have been possible in a society where adultery was treated more seriously, with actual penalties on the adulterer for breach of contract, but that idea's just so puritanical nowadays. That's not so out of line as it may seem, however, as marriage is a contract, not only between the individuals, but, arguably, with the state.) And since, as a group, lesbians are less promiscuous, this sort of argument only addresses half of the equation.
The long and short of it is that while Joe Carter may be right, it won't affect gay marriage arguments unless he can show that gay marriage proponents are advocating open marriages. Otherwise, the statistics are just statistics, and don't prove that homosexual men are unsuited for marriage any more than the 60% divorce rate proves that heterosexual men and women are unsuited for marriage.