Your comment is going up and down. Maybe this is because of the word marriage.

Formulated differently, from my subjective, unique, non reproducible and totally unscientific experience of being 16 years together with my wife with three kids, the quality and fun is positively correlated to the level of reciprocity and trust in the relationship.

And for the young readers, reciprocity and trust can really increase over the years, which increases the fun, which naturally increases reciprocity, which ...

I know you're getting dv'ed but I actually like this comment. It may have something to do with the true answer to the question. Perhaps developing social structures were somehow important or perhaps sex-as-fun was important to developing social structures.

Thanks, and by the way this is honestly how it has worked for me.

Just wanted to chime in I like this comment too.

Having a dedicated partner with whom you can explore your wildest fantasies is a very fun thing. :)