I think eventually people in general will start to become more liberal in the sense you are describing.
Our lives today consists in a level of interaction that we didn't have before, and everyone is constantly meeting new people and sure enough, some of those new people will be very attractive.
The tendency for me is clear and as time pass having open relationships will become more popular, and who knows, maybe because of this the divorce rate might start to get lower.
I still think many people are too opportunistic and we aren’t ready as a species, but I do think we need to start trying to step it up and give each other more freedom, or at least to try.
I really want to emphasize the idea that we don’t need to act on all impulses and using our own judgement and self-control is more valuable than feeling we MUST not act. It’s the same as something like self voting. We could all do it 10 times a day but then we wouldn’t have an awesome platform. Because we have this freedom, we can actually see more accurately how people really are and give our support in a more informed way. I hope I’m illustrating the similarity well here.