Well now, I can't speak to churches/temples/mosques/synagogues/oak groves but when it comes to the *legal* parts of marriage...

1) It's a civil contract. Nobody gets to look into anybody's pants when people make a civil contract.

2) (thread creep) I think they should be 7-year contracts, renewable by default (if you do nothing it gets renewed; if you opt out, it doesn't). The 7-year itch has a biological substrate - it takes that long to conceive and raise a child to the point where formal education (in schools or fields, depending) takes over from parenting as the source of primary care- and education giving. As long as all kids keep two supporting parents it isn't critical that they [the parents] live together as long as they get along... which is more likely than after a nasty divorce with lawyers. Kids with multiple homes may actually have an advantage.