The short answer: I think that, constitutionally, gay and lesbian couples deserve the same rights as heterosexuals. From that perspective, I'm all for it.
The (short) long answer: I am not against it, but I think that it is a very complicated subject. In college, I took a philosophy class titled Love, Sex, and Marriage. It didn't turn out at all the way I expected a class by that name at a small, religious university in the Midwest to turn out. I expected what I think most people expect from a religious institution when those three topics are brought up. My professor, a Protestant who'd turned to Catholicism, challenged the way I considered relationships in a really great way. My ideas of marriage changed. I wasn't against gay and lesbian marriages going into the class, but I didn't know they "why" of it. What follows is a short summary:
There can be no denying that a male and a female partner are needed for procreation. So, if procreation is the intent of marriage, than homosexual marriage is pointless. However, we are not animals who's only concern is the continuation of our species. We are emotional beings. We are attracted to people who meet our needs best. Some people find their needs, whether sexual or not, met best by someone of the same gender as themselves. If an individual has been "born gay", as some say, than homosexuality is a natural path for them. Although, if some traumatic event, such as sexual abuse or abandonment, led to the person's feeling of homosexuality, than I believe that they ought to seek counseling and therapy for that event before deciding if their sexual orientation is a part of who they are or if it is a reaction to an external occurrence. My heart hurts for people who have suffered in that way. I don't think that you can un-gay someone through therapy, but I do think there are sometimes other issues to be cared for, much like premarital counseling for a heterosexual couple. A lot of people, whether gay or straight, have hang-ups that need working through before they are in the right place to commit to a long term relationship.
Until science can confirm whether or not there is such a thing as a gay gene, or what have you, I think this will remain a touchy subject. Even then, some people will not accept it. I won't be one of them. Like I said, this is the short version of a very long and complicated idea.