Marriage is really a social contract between the partners and society as a whole. As hippies from the 60's and free spirits today will tell you, no one needs a piece of paper to cohabit, or even to have children together. In many poor countries, and in the past it was common for people to simply live together without the expense or bother of a ceremony. This was recognized in the concept of common law marriage. So why bother? Why does the state even need to get involved?
Because marriage is good for society as a whole. Unattached young males are a tremendously destabilizing factor in any culture. Marriage gives a format for young men to take on important roles in the family, and settle down. Marriage gives a structure for two people to work together toward common goals, and to take care of each other, reducing the burden on society, or the state. People in secure settled relationships can contribute a lot more to the community in which they live. Single people can become a burden when they become ill or old. Most if not all cultures recognize this and create incentives and support for marriage.
What's the problem with gay marriage then? It provides stability, structure, and all the benefits of heterosexual marriages? There is no logical reason to deny gay people the benefits of legally sanctioned and supported marriage.It just makes sense.
Religions often denounce homosexual relationships because those relationships tend not to be as fertile as heterosexual relationships. Religions rely on reproduction and proselytizing to survive and propagate. Religions that don't emphasize fertility tend to get out-competed by religions that do emphasize unrestricted breeding. Mormonism, Islam, and fundamentalist Christianity grow so fast in large part because they restrict women's roles to childbearing and therefor have lots of baby Mormons, Muslims, and fundamentalist Christians. While it's great for the religions, this is not so good for the individuals, or for the society we live in today. Today our kids need access to good educations, and it's darn hard for the average family to send 6 or 12 kids to college. Overpopulation is a serious concern for our whole planet; energy and resources are rapidly becoming depleted. It's better for our society if most people have one or two kids, and take really good care of them. Gay families that don't reproduce, or adopt, place fewer demands on limited resources.
There's no reason to discourage gay marriage anymore. It's actually a really good thing for society. Thank goodness most of us are coming around to see that. Yay Utah!