Idealized, traditional heterosexuality told us that you were not to have sexual relations until you were married and then you were supposed to stay monogamous. If everyone followed these guidelines (I know most don't, but humor me) there would be virtually no sexually transmitted diseases in the heterosexual community.
The homosexual community has no such guidelines. They have no moral structure to their relationships. There are no rules saying you should remain pure until you find your same-sex life partner. There is no strong tradition encouraging gay couples to stay together for life. In fact one could point out that the gay culture encourages experimentation and open relationships. I recall a gay comedian talking about being broken up after his boyfriend of six months moved out. "That's the gay equivalent of a 50 year marriage for you breeders!"
Even my gay friends (I live in Fort Lauderdale and have LOTS of gay friends and acquaintances) say that gay marriage is not desired by most of their circle. A few would like the option, but even their comments of "how would you handle divorce" and "as soon as you do, you'll meet the man of your dreams" speak to looking at marriage as something that would be designed to be exited.
My main concern is that this will further water down an already beleaguered institution and cause it to become completely meaningless. I cannot imagine that being good for our society in the long term.