So let me test my understanding.
I’ve read people saying that they knew their SO would be hurt by their cheating, but that they wanted to so they did it anyway - regardless of the hurt they knew they would cause. They term this behavior as “selfish”.
Eh.
No.
Not “selfish”. Selfish is taking the last cookie in the jar or always demanding to be the center of attention. Deliberately behaving in a way you know will hurt a person you purport to care for isn’t selfish, it’s abusive.
“I know it will hurt if if I hit you with this brick, but I want to, so I’m going to. Oh, my goodness, how selfish of me!” Would you punch your SO in the face if she didn’t let you eat that cookie you wanted? No? Why are you willing to make your SO pay in pain for your moment of pleasure then?
I’m unwilling to argue whether monogamy or polyamory or serial monogamy or any of the staggering variety of sexual behavior patterns available is the “natural” one. For purposes of discussing cheating, it’s irrelevant. A big, fat crimson herring.
“Cheating” occurs when one partner violates a promise to the other. Either an implicit or explicit promise - and if you’re going to attempt the “but I never actually said we were exclusive! My SO just assumed that and I let them!” argument, just don’t - deliberately manipulating your SO’s assumptions to get what you want is equally deceptive. It’s about lying and deceit and betrayal. If you don’t think monogamy is the natural choice - or the correct one for you, personally - then by all means say so. To your partner. At the beginning of the relationship - not after you’ve already started cheating (or have gotten a golden opportunity to do so). If you agree to monogamy and then violate your promise, don’t try to cover it up with psuedointellectual masturbation by saying “Oh, but monogamy isn’t natural!”. That would officially be not the point. The point would be you broke your word - or you lied to your partner in the beginning - without considering their pain. Or by deciding that your pleasure is worth inflicting pain on them.
For me, cheating isn’t really about the urge (or even the actual behavior) to have physical intimacy with another person. It’s about the betrayal of trust and the inevitable lying and deceit. I don’t care if my SO occasionally has the desire to have sex with other people - it would be unreasonable of me to assume that all his desires are centered on me forever. If he’s willing to lie to me and break the promises he’s made to me to get it, that’s another story.
I expect my friends - and most especially my SO - to be honest and considerate people. I expect those close to me to refrain from hurting me on purpose. In my turn, I will be honest and considerate and refrain from hurting them on purpose. It works both ways.
This is not even mentioning the very real point of “let’s not infect our partner with communicable venereal disease that could kill them or render them infertile” that is the inevitable corrollary of cheating. If I can’t trust you to honor your word, why the hell should I trust you with my life? If you can’t be arsed to take my well-being into consideration when deciding to bang anything that catches your eye, why should you be arsed to use protection every time?
This is not to say infidelity is always wrong - there are precious few absolutes in human interactions. But in the absence of further information detailing to me why in your specific instance betrayal, dishonesty and deliberate causing of pain were warranted, I am going to think less of someone who cheats. Nor will I trust them. Why should I? They’ve already demonstrated they are untrustworthy.