Traditional marriage (one man+one woman), as an institution, has intrinsic human value and benefits society, regardless of whether or not it's sanctioned by the State.
Most get their basic understanding of themselves, their gender identity and how to relate to the opposite sex from observing both mom and dad. Remove one during childhood, at such a crucial time of development, and it's not surprising to see anger, various forms of arrested development, along with sexual identity issues.
Unless they're overcome, these things are then passed along to the next generation in a vicious downward cycle.
For these reasons, it's in society's best interest to sanction traditional marriage, imho.
I believe this is related to why children and younger adults tend to be so screwed up these days, and by extension, society over all. I think it started with the children of the 'baby-boomers', and has really picked up steam.
When I was in high school during the 90s, I knew ONE person that still had both of their original parents married and living together. Right now, I'm the only person I know out of all of the people I knew and went to school with that is still married to the same person with our children. My wife and I married very young, and were COMMITTED to giving to our children(when we eventually had them) that which we never had....A functional family.
People are so morally bankrupt. Seems every week I'm hearing of married people engaging in infidelity, and families breaking up because of it. Not only are they betraying their spouse, but their children as well. It isn't only the men either. If anything, from my observation, women are every bit as bad if not worse. It disgusts me how people can make such a permanent mistake that will breakup their families and affect their children forever, all because of some physical attraction or sexual adventurism.