I think it's extremely important that all partners are informed that the person or both have sex with other people. Then it's fine.

I don't believe it's anything to do with society. If particular couple is ok to sleep around, then there should be no guilt or judgement. It's their choice.

But it's a different story if people aren't aware. Then we denied them free choice. Personally I don't want to be with a man who sleeps around. So if he informs me, I have a choice of staying or leaving. If he doesn't inform me, then he denies me my free choice, he keeps me around under false pretenses. It's not cool. It's not cool in the same sense as not informing your date about being married or having children etc fundamental stuff.

There is nothing liberating about lying to people. It's selfish. But if both are truthful then of course it's entirely their business.