What if there's a world where EVERYONE does Sex ONLY for fun - all pregnancies are either prevented or aborted. We're looking at extinction then, no?
Or of course, we're looking at the propagation of only those ideas and mentality which believes in institutional procreation.
In short, those who have sex only for pleasure are destined to phase out faster than the "species" intending procreation and transference of culture and practices to future generation.
(Am just thinking TOO mathematically and long term)
But here's another interesting question. Who would you prefer to have sex with, someone who's exclusive or someone's who's had a few other partners?
Personally, I can see a lot of avenues of stress/depression/anxiety opening up when you remove the bounds of long term partnership.
And adding another note here - Marriage isn't only about sex:
This Recently Married Man Just Realized Marriage Is Not For Him