It's not about the sex. It's about trust.
When you engage in a normal relationship, you devote yourself exclusively to your partner, and they to you. It is a choice the both of you make for love; a binding of trust, if you will, and one that is completely voluntarily.
If you don't feel like you can have such a relationship, then maybe you should try to have an open relationship. This is a relationship where you devote yourself emotionally to only one person, but you're allowed to experiment outside of your relationship as long as those experiments are not love-related.
Then there's also polyamory/polygamy. In this relationship construction, you are free to build up a relationship with multiple people... Which, in my eyes, kind of defeats the whole purpose of relationships entirely, but hey; some people do well enough in such relationships.
If you can't remain faithful, then there's still no reason to cheat on your partner. There are three solutions which are much better:
-Breaking up before having sex with someone else. This is the honest way of doing it. You're not breaking their trust.
-Opting for an open relationship. At least you'll have consent, your partner is aware of your actions, and you offer the same to your partner.
-Opting for a polyamorous relationship.