It might be that even though marriage is nowhere mentioned in the Constitution, it may be considered as a strong enough tradition to warrant constitutional protection. If that is so, then I cannot see any way the Supreme Court could hold same sex marriage as any less legal than opposite sex marriage, or interracial marriages either. A strict, originalist interpretation could say otherwise, so I guess we'll see. It is possible the court will see this issue as not so different from the abortion decision; if it ain't in the Constitution then it ain't constitutional and the US Congress would not have the legal standing to pass a law to regulate marriage and the issue could fall to the states just like abortion did. There has to be a line that delineates what can be legislated at the federal level and what can't. For a long time now the federal gov't has stretched that line quite a bit, but we'll see what happens. My own personal feelings are that every American ought to be able to enter into a marriage without any gov't regulation whatsoever, except for age and mental capacity, just like you can for any business dealing or anything else. A person ought to be able to make their own decisions, but the question is whether the federal gov't should have any say in that. Purely from a legal perspective I'd say no, it seems to me that the question should fall under the 14th Amendment, no discrimination allowed.
Will Congress pass the Same Sex Marriage Act? I hope so, but it's foundation in law is somewhat suspect. And I might add this: people can get married without the blessing of any law or level of gov't. The couple can create every legal document that a marriage assumes, no matter what their circumstances are. Wills, Power of Attorney, Physician's Directive, stuff like that and more. More or less like a business contract, if I can enter into a business contract with anybody of any color, creed, or gender identity then that is essentially a marriage in every way except the way your gov't looks at it. It shouldn't be that way, but right now it is what it is.