Cheating isn’t wrong because your partner owns your body and gets to decide when you have sex, it’s wrong because you’re breaking an agreement you made with your partner.
Monogamy is an agreement that people in a relationship should talk about openly and explicitly and decide on together without pressuring each other. If people haven’t agreed to be monogamous, having sex with someone else isn’t “cheating”. If people have agreed not to have sex with others outside the relationship, it’s wrong to break that agreement, just like it’s wrong to break any other agreement you make with your partner.
Like any other agreement, some people will think this is very serious and others won’t mind as much, but it’s still important to keep any agreements you make with your partner unless and until you inform them that you don’t plan to keep that agreement anymore.
It’s not kind or respectful to sneak around and not allow your partner to make decisions about the relationship based on current information instead of old agreements they assume you’re still keeping.