Marriage and monogamy was covered in a recent profile of Dan Savage in the New York Times Magazine and also on the Colbert Report. I know it's easy to conflate the two concepts because like most people, I've been taught to equate the two; however, they are really two separate issues.
Let's start with marriage. I can think of these common reasons a couple would get married: 1) a bundle of legal rights and responsibilities and 2) an expression of love and commitment. First, marriage is a legal contract that includes among many other things: inheritance rights, making health care decisions, responsibility for debts, and tax benefits. Second, marriage is also a public expression of commitment to each other and a community. Notice, these reasons are the exact same reasons why a same-sex couple would want to get married.
For same-sex couples, however, people have created the concept of a "civil union" to refer to the bundle of legal rights. Many people say they support "civil unions" but not same-sex marriage. Why? If they are truly equal, then why not call them both simply "marriage"? The only logical reason I can think of is prejudice. It keeps lesbians and gays as second class citizens. Therefore, if you support equality for all, you should support same-sex marriage.
Getting back to the topic of monogamy again. When I used the word commitment earlier, many people would assume a monogamous commitment. I believe Dan's position is that this assumption of monogamy should be examined more realistically. Whether you're married or not, you can be either in a monogamous relationship or not, and this can change with time. After all, there are many examples in the news lately showing that humans aren't that good at monogamy, especially after several years, let alone a lifetime. If we are able to accept reality and be open and honest in our relationships, then perhaps there would be less suffering caused by unrealistic expectations and less pain inflicted on families that are forced to break up as a result.