Easy to be rational when you're not going through it.
Only because, when you're going through it, you don't think rationally.
This is the reality of it:
The person did not care for you enough to fight off the temptation. And they didn't respect you enough to Donell Jones themselves out of the relationship. So clear your mind, and don't contact that person.
The lack of contact will hurt them more than it will hurt you.