Here's my opinion:
If you know for sure the other person won't accept you having sex with other women, doing it on her back will end up spoiling your relationship in other ways.
Why don't you try to make something different? Explore eachother fantasies, those you can perform together? You may even improve your cumplicity. Be a little crazy.
Plan something strange together.
Time does not imply evolution. Very true. We are stepping back. One generates haunting monsters that generate haunting monsters on an endless spiral of misunderstanding, unsolved needs, moral amulets eradicating both the sickness and the cure.
I see a bunch of men raging at the void, haunted by their own inventions. Absurd. Totally absurd.