Why is "cheating" considered "bad" even if you never plan on leaving your partner?

Undermines trust and invalidates all words and expressions of love associated with uniqueness. Weeds in a rose-bed.

Cheating is bad when one person in a relationship decides that it's an open relationship when previously, the relationship has been built on trust and honesty about the relationship being monogamous and the cheater hasn't discussed the new rules of the open relationship with their partner.

I agree with all you say. First of all human nature is not naturally monogamous. Monogamy is clearly taught. So I support people being free to have sex with whoever they want, and take care of themselves of course, and I love the idea of friends with benefits. But here is an interesting scenario: Sometimes my friend with benefits winds up getting a “real” girlfriend and then screws me over and doesn’t even respond to my texts. Now that, I don’t like. Be free but if you are my friend, be kind to me, treat me like I have worth as a human being and like our interactions were meaningful. I agree with you, sex should bring friendships closer, not further apart. But alas, human nature, unpredictable as fuck. Thank you for your posts and sharing your mind!

Difficulty Level 3: Commit to a single partner.
Difficulty Level 2: Have a partner and cheat on them and then leave them.
Difficulty Level 1: Have a partner and cheat on them and intend to keep them whenever they are near you while cheat on them when they are away.

People want to have those people in their lives who would strive for them.

It's not so much to cheating but the gang of cold sores breaking out on an innocent person's face

... Until they leave again, eh? ...

But will you tell them that you have cheated on them??

If its not bad, tell your spouse or partner openly.

Because it is bad, whether one does it or not.