Should they? Up to them I say. It already happens, men sleeping with other women, women sleeping with other men, most of the times, the partners know they're cheating on each other too. They just turn a blind-eye to it. Some people even share their partners voluntarily (Swingers). Whether it works, depends on the type of relationship between the couple.

The sexual repression i'm talking about, is not about "moral values". It seems you totally missed the point. The repression is the CAUSE of all these problems to begin with. You can express your sexuality, you can be active and indulge in it... all while maintaining and respecting the sanctity of a relationship. The keypoint to understand here is that to do this, you need to understand  and be in-touch with your sexuality. If not, you won't know how to handle it, you won't know what you want/need and thus you will always want to "explore", guess what that leads to?
My my... bitter aren't you? are you really any different? We are all human, we all have a degree of selfishness, and when it comes to love, it is inherently selfish. You have to care for yourself first before you can care for your partner. If your own needs are not met, how can you meet the needs of your partner? A relationship works when both sides fulfil each other's needs.

I don't know why people like you are so pissed about stuff like this. Sure it hurts the other party, sure it isn't the "right" thing to do, but you know what? Its irrelevant. Those who are hurt will move on, its a sign that the relationship wasn't meant to be in the first place. Those who cheat will get what's coming to them anyway, and sometimes the cheaters turn out to be the better match. I rather people make these mistakes earlier than later.