Interesting concept. If you've agree to be non-monogamous with your partner, then fine. Otherwise cheating is deceit, and if you continue to have sex with them when they would leave you if you were having sex with other people, then thats not informed consent. It also risks STDs.
But as for the concept of romantic love, I guess throughout history, people got married for many other reasons - power, money, status, to unite kingdoms or powerful families, etc.. And then the wealthy in society (the so-called "upper class") often had affairs and mistresses (and often still do).
As for the rest of us, I think theres very little reason for women to be in relationships with most men if they can't offer loyalty and therefore reliability. Especially in modern times, when women can earn their own way and have babies on their own. So the average man has to offer traditional romantic love to give the woman a reason to be with him. If all he offers is sex, well thats easy enough for women to get without having to have babies and binding themselves somewhat to one man.
Thats the theory. Of course you get men who have children with several women, and vice versa. It must use up a lot of energy to do that though - I mean if you want several sexual partners, you need to spend a lot of time finding them and keeping them happy enough in order to keep them agreeing to have sex with you. That doesn't leave much time for making money or other hobbies.
Its mainly a Christian concept, isn't it? Monogamous marriage and now relationships in general. I guess at some point, people sat down and worked out that monogamous marriage was beneficial to society in general, because it provides stability, gives children a stable upbringing usually, and doesn't generally produce too many children. It gives most people something to hope for, to work towards, a goal in life.
Its also quite good for career prospects. There is a reason politicians try to project the marriage and children image. It seems stable, reliable, you think they're one of the good guys or girls, they're not spending their time sleeping around and hunting for sexual partners but are working hard in their jobs as other people are in their careers. And its good for men's health too, and often for their behavioural and social skills.
Even in non-monogamous societies, its only a few men who have more than one wife I believe, and then generally only the wealthiest men. I've never heard of a woman having more than one child.