The difference here is that when you are not committed and you are hooking up or trying to with “Na-mehrams”, it is between you and GOD but when you are committed or married and you are still trying to hook up with someone else, then you are cheating your spouse and you are cheating the other person too(if you are posing yourself as single), and this way this doesn’t only remain between you and the GOD. And that is why treating both cases as same has no justification.