The concept of marriage is based in religion, and it's sole reason for being is to establish a set of morals and expectations surrounding both responsible sexuality and the raising of children in a healthy environment, as its set forth by God.
In that sense, a relatively small percentage of people enter into marriage with that kind of commitment and approach. Many just think they want to play house, or they get pregnant, or they want to secure their mate from a threatening foe, or they just do it out of convenience. There's really no "life" commitment taking place; it's more of a tryout than a promise.
Therefore, divorce is a very real (and reassuring) option. Truly married people don't see it as an option at all.
It's an impossible argument to make that gay marriage is about child-rearing or a religious sexual commitment. It's a deviation from both.