I'm not going to punish my son (21M) for repeatedly cheating on his GF
Are you sure about that?
Why would I punish him. Everybody is entitled to learn from their mistakes.
That's a reasonable attitude to take. While children sometimes need to be taught lessons, your son is an adult, and if he chooses to cheat in a relationship, then that is not your concern.
I'm saying he didn't make any mistakes by cheating, he isn't in any real relationships.
I'm not sure what you mean by that, but if this relationship was not serious, then that is another reason to not punish your son over it. He has made his own decision, and he should live with the way that it turns out for him.