Although there are a few states that have recently begun to recognize same-sex marriages, most do not (see: Where is Gay Marriage Legal?). Of those states that don’t however, some do recognize domestic partnerships, including those partnerships formed in other states. This means that while your marriage may be considered legal in your current state, other states may or may not respect the marriage even if you participated in a traditional wedding ceremony.
Because legal rights and benefits are so questionable for same-sex couples under the current state of the law, it may make sense for you to obtain a domestic partnership even if you are legally married in a state that recognizes same sex marriages. The domestic partnership could provide protection of your rights as a couple should you decide to move to or visit a state that recognizes domestic partnerships but not same sex marriages.
In addition, because the laws surrounding same sex marriages are constantly changing, it certainly doesn’t hurt to have as much protection in place as possible.
This would be especially true in the event of a divorce/separation or death of one of the spouses as these are new and unchartered legal territories without solid rules and regulations in place.