Imagine for a minute that marriage never existed.
Fidelity/infidelity as a result, also never existed. Imagine that the wishful thinking of Jean M Auel in the Clan of the Cave Bear and the rest of the series Earth's Children, is actually the norm. In these books, from the time a girl becomes a woman (from her first menstrual cycle on) and the time a boy becomes a man (after his first kill - about the same age as for girls), they can just have sex with whomever they want, whenever they want. Babies are gifts from the Mother or from the Great Cave Bear, depending on whether you are homo sapiens or Neanderthal, and they have no idea that sex is involved. At least in the books, "mating" does exist. If you are Neanderthal, your mate is chosen for you and you are stuck with him/her until one of you dies. Homo sapiens also mate but some do separate - although this is rare. So families are more or less stable, children are brought up by the mother and her mate.
Yet these same people (one of the Homo sapiens tribes) plant acorn nuts in the place of one of the huge trees they fell to make their boats. So they understand that the tree came from a seed, but they don't know that babies come from sex?
Without delving into the books further than that, (which, apart from the wishful thinking about sex are actually pretty good stories and well-researched and informative - not for children though), imagine that babies are gifts from a creator (or in our atheist day - from nature) and noone cares who the biological father is. Children are brought up by whomever is the the current partner of the mother.
Never mind the jealousy issues which would still be there, even though noone would be "allowed" to express it, in a polygamous society, (in societies where one man has more than one wife, there are jealousy issues between the wives, despite the fact that it is a cultural norm), imagine the biological issues this would create.
If noone knew who their real father was, the chances of having sex and having children by one's half-sibling or first cousin would be pretty good, and this would not be the exception, but the norm. We'd probably be extinct by now, just because of all the health and mental problems our ancestors would have had.
Even if we did manage to survive, we'd be pretty screwed-up. With no such thing as marriage or fidelity, the only stable people in a child's life would be his mother and maternal grandmother. There is enough out there, written by psychologists, on the effects of being fatherless on children, that I don't have to add anything here.
So we have a human race that is plagued by health and mental problems as well as an extremely high criminality rate. Sounds like a good place to live right?
And some people believe that marriage is just a ball and chain for women.
Oh, but you can be true to one partner without marrying, some would say. Yes, but that would just pretty much be unofficial marriage. The idea of fidelity exists because marriage exists. If marriage had never existed, then why would fidelity?
Marriage exists in order to create stable homes for children, allowing them to be brought up (ideally) by their biological parents, or at least by one father and one mother. It does NOT exist for the sole purpose of allowing a man to have a female slave, and that kind of thinking does need to be changed, not by abolishing marriage, but by bringing it back to its original purpose.
My own marriage as taught me that it is possible (and not wrong) to be attracted to people other than one's spouse. Attraction is neither good nor bad, it is what you do with it that is good or bad. It is also possible to love these people, or like them very much, and that is not wrong either. Being married, I am no longer available for any other exclusive relationship. The fact that I am unavailable, has given me the opportunity to become friends with a couple of people who might have been "more than friends" had I been single when I met them. I have discovered that as you get to know a person, eventually, the initial physical attraction does wear off. And quite often, you discover that you wouldn't want to be with them anyway.
If only I had known this when I was younger, I might have saved myself some heartache. I thoroughly promote the "let's be friends first" method of dating (or courting, for those that are into that), because it's a lot easier to just "stay friends" than it is to break up and lose someone who could have been a good friend.