No its not the same. If someone's SO has already basically ended the relationship in substance if not in form, then he/she probably doesnt care other than ego-wise that the other person is stepping out. There's no betrayal, there's no devastated feelings. I'm talking about where 1 person is happy as a clam in a relationship and his/her significant other cant keep their pants on outside the relationship.
And a pattern cheater is different than a special case "cheater" that you describe. The pattern cheater is the leopard.
I've known a number of girls (& been involved with a few) who _always_ have 3 guys in their lives emotionally and sexually: The immediate ex, the current bf, and the future potential bf. That sucks.