[carlob]

You might be right morally, but in a lot of countries it is still the case that 'not having sex with other people' is a compulsory clause of the marriage contract. So legally it's a little bit murkier.

People have been advocating for marriage contracts that are a bit like the CC license, where you can pick the parts that you agree on.

[mgbmtl]

Marriage is on its way out in many places. In my region of Canada (Quebec), less than 50% of couples with kids are married. That number keeps going down every year.

(There are some automatic protections in the event of a separation.)