Our species has a real problem with sexual relationships. Other animals seem not to have this issue. They just do it with whomever they want and they deal with the consequences however they like; they go with instinct and their instincts more or less follow the pattern of nature.
But we, on the other hand, have this thing called a ‘developed’ mind, which creates all kinds of issues. For instance, attraction is instinctive, but there are so many considerations that we can’t follow through with just instinct. If you are committed, you STILL get attracted to other people but you pretend not to. You try not to act on it. This is because our society needs stable units (couples, family) for order. That’s why we’ve created a sense of possessiveness over our partners and religions with crazy commandments. So, that’s probably why 50% of couples have affairs and the divorce rate is rising (you have to hide your instincts, but you can’t hide your affair forever). Those people who are NOT having affairs are probably past being repressed and have reached a state of true contentment; these people are few and far between.
It’s really a lot to do with sex. We love sex because we love feeling desired and forgetting our limited realities. There’s nothing wrong with that unless it becomes an addiction. It’s ok if we do it, but we can’t tolerate our partners being attracted to someone else. This is pure ego; our little self feels threatened. Also, we need to keep a false sense of order in society by thinking that ‘free sex’ will result in STDs spreading like wildfire; this is an important consideration, but will making things TABOO actually stop them happening? More likely, those instincts will STILL manifest but in a worse way, because they happen out of desperation and guilt. In a society of open education and informed freedom with regard to sexual relations, there are likely to be fewer negative consequences.
So, I think we should all just chill out about sexual relationships, be honest, open and informed. Two consenting adults who engage in a relationship that is non-possessive are more likely to be independent and fulfilled rather than desperate and needy.