Cheating? So will have NTR?
Not a very thin line at all. NTR = You see your love interests hooking up with someone else. Cheating = you hook up with someone who is already in a relationship. In the second case, there's no NTR as long as the existing relationship either ends after meeting the MC or isn't a romantic/sexual relationship (e.g. a marriage for money or power).
The only time it's a grey area is when that existing relationship remains ongoing, so that you don't know whether the love interest might be doing off-screen things as part of that existing relationship. It's arguably not NTR, because nothing is being shown or talked about, but it is at least a grey area.