Maybe I have a different way of thinking about things but I don't think it's morally wrong for someone to have sex with someone they like outside of their relationship. This has everything to do with possession and ego. We're conditioned to think of relationships as an ownership type thing. To me if a women wants to give someone her p*ssy outside of her relationship, it's her right because it's her p*ssy. If we're speaking from a Utilitarian aspect of morality and ethics.....cheating promotes the greatest happiness of the greatest number of people (2 vs 1). If my girl were to cheat on me (with or without me knowing) it really doesn't "hurt" me.....It just hurts my ego.
That said, if you make a vow you should honor it......but that's on them and not me.
That's a very logical way to go about it. But let's be honest... we all would feel really bad if our significant other went with someone else. Because usually it means, they don't love us anymore. Humans, at the very core, just want to be loved and appreciated. So, when someone you would think cherishes you more than anything else in the world, leaves you for someone else and sleeps with them, it hits you to the core. Now, we could argue that this is society's manipulation of how relationships, dating and population control work, and you could make a very strong point for it. You're right, it's their body and they have every right to do what they want. And likewise, you have every right to feel the way you do, because relationships come with the pretense of it being exclusive between you and them. Otherwise, why not just keep it to dating exclusively, or harems like what they used to do way back in the day?
And let's not forget.. that ego you mentioned is a dangerous monster if left uncontrolled. Because it's that same ego that can cause narcissism, greed, gluttony, lust, and hatred. And when you hold all of that in your heart, you will not act within the purest of intentions. You will act from a perspective of revenge. Now, given your response, you sound like you have your ego in check and will probably be able to be just fine. However, a lot of people, men OR women, aren't exhibiting the same self-control. People get killed for much less. Look at how much violence is caused over infidelity or untrustworthiness. A perfectly rational man can very well turn into a monster if his emotions and ego is currently going into a state of panic and frenzy. Humans can be a slave to emotions. How we feel. How they feel. It's the core of everything. So when a lot of people are saying they don't mind ruining a relationship, or otherwise known as a spoken or written bond between two people... there's no surprise that the individual on the receiving end of the betrayal and disrespect will react in violence or otherwise unfavorable actions toward you. We COULD encourage people to have multiple partners, but considering those values are ingrained into human behavior itself, there would be multiple problems, and therefore we're a monogamous society.