Sex is fun for the same reason eating and breathing are fun: we wouldn't do it if it wasn't because we're too lazy. Evolution designed our sexual behavior to be consistent with our biology, ecology, and our social structure, with no doubt, some feedback involved. As long as we were hunter/gatherers moving around in small bands in an empty world, it probably worked fairly well.

When we invented agriculture and large, permanent communities, we couldn't evolve fast enough to alter biologically to meet the new social conditions. Culture then evolved to manage our reproductive activities. It wasn't necessarily fair, or even fun, but workable methods of sexual and family management survived, while those that didn't vanished. It was purely natural selection.

Today, society changes faster than culture can evolve, so we are going to have to change again, perhaps by deliberately altering our culture through political action. After all, isn't that what child support and divorce courts are all about?

My guess is we'll wind up with a mixture of our old traditional marriage customs and a legal system and social net to try and correct the damage when the former no longer works. This is pretty much what is happening now. The system is under stress, particularly in the advanced countries, and the number of dysfunctional families and screwed up kids is increasing. This isn't the only place where human biological and cultural evolution cannot keep up with changing conditions, and the results are crime, displacement, poverty, civil strife and conflict.

Humans are infinitely adaptable, we'll probably work something out, but there will be friction and damage to the system, and a part of our resources will have to be diverted in order to keep things operating semi-smoothly. We've always payed a price for "progress", and there has always been nostalgia for an idealized past when everything was supposedly better. I suspect the future will be no different.