Since when do marriage and monogamy go hand in hand? Obviously, they should, but a piece of paper signed by some damn clerk isn't going to stop someone from cheating on their spouse.
FWIW, there is far too little monogamy in traditional, heterosexual marriages. Why would gays fare any better?
This is what Andrew Sullivan has to say about homosexual marriage and monogamy:
Andrew Sullivan, a homosexual activist writing in his book, Virtually Normal, says that once same-sex marriage is legalized, heterosexuals will have to develop a greater "understanding of the need for extramarital outlets between two men than between a man and a woman." He notes: "The truth is, homosexuals are not entirely normal; and to flatten their varied and complicated lives into a single, moralistic model is to miss what is essential and exhilarating about their otherness." (Sullivan, Virtually Normal, pp. 202-203)