> Most other cultures were the same way. Everyone having sex with everyone made for a pretty bad society.

The opposite of true. I recommend "Sex at Dawn" for a good treatment of this topic

https://www.amazon.com/Sex-Dawn-Stray-Modern-Relationships-e...

TL;DR "Everyone having sex with everyone" pretty much maintained social cohesion in small societies for millenia.

>> How about couples in a monogamous relationship - is that natural?

There's some evidence that while short-term(a few years) monogamy is natural , long-term one isn't . this book talks about it:

http://www.amazon.com/Sex-Dawn-Stray-Modern-Relationships-eb...