OTOH, a monogamous relationship, incl. marriage, is more-or-less-implicitly a contract where you agree --among other things-- not to have sex with anyone else

Cheating is quite common; about 20% of married people have affairs and the rate is higher in putatively-monogamous unmarried partnerships. Around 3% of children are the result of affairs.

I'm not sure what other sorts of "contracts" have a 20% chance of default. I don't think banks would offer you a loan if they thought there was a 20% chance you wouldn't pay up. Even Florida's foreclosure rate isn't that bad!