Lifelong heterosexual monogamy actually works for a few people, and it's nice when those people are happy, but trying to make it the standard that everyone has to live up to hurts the majority of people who just don't work that way. The more strictly the standard is imposed, the more people are hurt. Far from being an ideal that an englightened society should embrace, traditional marriage is something that we should be trying to put behind us with the sort of cultural shame that we now apply to traditional practices like trial by combat and the divine right of kings.