Consider this. I have a friend who lives with her boyfriend and has for years. He wants her to marry him. Not because he wants to make a lifetime commitment. It's because he wants his girlfriend to have his medical benefits and other benefits that a wife has. Sanctity of marriage? Seriously? People are getting married so they can have these special rights being given to married couples (rights and privileges not given before the 1900s, so you can't say this is traditional). Unfortunately, we live in a part of the state that didn't recognize benefits for unmarried couples. Some people in this state do live in those areas. Giving domestic benefits to unmarried couples (gay and straight) not only satisfies some of the desires for gay rights, but it actually helps the sanctity of marriage because you won't see people getting married just to have benefits.
This fight is mostly about a bunch of special rights given to married couples, and they didn't even have these special rights before the last century.