That bastion of conservatism, the New York Times, reports that “monogamy is not a central feature for many” gay and lesbian couples.
In fact, fully 50% of gay couples are involved in polyandry, according to the soon-to-be-released Gay Couples Study by San Francisco State University. (In contrast, between 1.7 and 6% of heterosexual married couples are in open marriages, per the last paragraph of section 2 of this link). A brief trip to Wikipedia lists dozens of sources confirming the frequency of polyamory among homosexuals.
Perhaps more troubling are the reports of activists demanding polygamy. England has already taken the step of giving benefits to multiple wives, as long as the marriages occurred in another country; meanwhile, in Sweden, activists have been calling for more flexible marriage laws (or even marriage’s outright abolition) for several years. (The Family Research Council also has a useful pamphlet dealing with many of these issues.)
Basically, though some people in the gay marriage movement want only the “right” to “marry” their partners, this legalization will inevitably lead to a radical re-definition of marriage that is harmful to children, and therefore to our very future.