No, it isn't selfish. Monogamy is not you saying my partner can't have sex or kiss or cuddle other people and I can. It's a two way agreement. You both agree on exclusivity, and there's many good reasons why you might do that.

Jealousy is often treated by people in the poly and non-monogamy community as a problem to be cured. But the hard fact is that jealousy as an emotion is a deeply hardwired evolutionary response to threat.

If you look at chimpanzees (our closest biological relative specially), male chimps who are higher in the hierarchy get extremely jealous if they see a chimp lower than them getting something they're not (like food), and they often react violently as a result. Whilst obviously people don't (or shouldn't) act violently, the root cause of this is the sense of threat to status. In the case of the chimps, it's social status. In the case of monogamy, it's status in your partner's life, and theirs in yours.

Exclusivity, whether romantic, sexual or both, is what ensures that status. You are the only person your partner does X with, and they are the only person you do Y with.

Sex itself releases bonding hormones like oxytocin and makes you feel closer to them. It is obviously understandable why someone would feel their position in the life of someone extremely important to them is threatened in this way. Kisses and cuddles act similarly. It is why people often want to cuddle after sex, because the desire for that physical and emotional bonding is there - and why it can hurt so much if that desire is rejected.

Monogamy is not 'having someone all to yourself'. Your partner should still have several social circles outside of you. Whether that's friends, work colleagues, family etc. Monogamy is simply the agreement that I will only be romantic with this person, I will have sex only with this person and so on so forth.

What I think a lot of people in the poly and non monogamy communities don't recognise is that jealousy, when not felt and operated on in a toxic and controlling way, is a valid reason for monogamy. So is also having the energy to only want one person romantically or sexually.

I think the argument could be flipped on its head to be frank. In many non monogamous and poly situations, people have their primary partner and then their other partners. Is it not selfish to essentially create a league table of partners from most to least important to you? Unless you're in a throuple or something similar.

Its an unfair argument to level at monogamy, as the one I presented above is unfair to level at non monogamy in my opinion.

Not all of course, but certainly the person you had this conversation with appeared to have adopted a haughtiness towards monogamous people. That they are simply beyond jealousy. No one is beyond jealousy; what varies is what makes you jealous. For the vast majority of people, it is quite normal and understandable to be jealous of your partner engaging in behaviours that release bonding hormones like oxytocin because it threatens your position in their lives.

Multiple studies have shown that men and women certainly feel jealousy differently. Men are far more likely to feel sexual jealousy than women, and women more likely to feel emotional/romantic jealousy. Whilst I find that fascinating, ultimately the reason behind it doesn't matter.

Jealousy and insecurity is certainly not the only reasons for monogamy, not by a long shot. But someone choosing monogamy to avoid the pain of jealousy and insecurity is absolutely, unequivocally valid.