Casual sex is good, or is it?

[Craterus]

Why not? couldn't you say the same thing only opposite? "If we were really concerned about the other person's happiness, people wouldn't cheat on their partner"

You are saying it is the person being cheated on who is being selfish? Hahaha
What is this? Take a common sense position and then twist it to be a clever absurdity day?

Cheating causes disease and breaks up relationships and trust. Having an open marriage is stupid but your choice to make, but cheating is something else. Engaging in risky and communicable behaviours while lying and endangering a directly interested party is the pinncle of selfishness.

Agreeing to an open marriage is a consented thing, yet rarely works - Cheating is lying and betrayal.

Then you admit that relationships have more to do with our own happiness than the other person?

[Evil_Maniac From Mars]

Then you admit that relationships have more to do with our own happiness than the other person?

Relationships have something to do with keeping both partners happy. If I cheated on my girlfriend, she would be angry (and I would be angry at myself and it conflicts with my sense of morals, but that's beside the point) and unhappy, and therefore I would never cheat on her. This is considering the other partner and keeping them happy, which is the point of a relationship.