Yeah, I'd agree most people get married thinking it's going to last and believing in that sanctity. But to suggest people never marry for other reasons? Nah. Green cards, money, girl got knocked up. I don't think everyone's reasons are as pure as you're suggesting.
And even if they go into it believing in that sanctity, certainly more than enough people don't treat it as such when they're married. I guess as someone whose parents had a downright shitty relationship, I have a hard time with the whole "sanctity of marriage" line of reasoning. And quite frankly, if
I believe marriage should be between a man and a woman only, then I just won't marry another woman.