[Jamie89]

I don't think the Mark thing is an excuse, even if she was sleeping with Mark. If he wanted to make a go of their relationship (which he did - he'd even told the girl he slept with that) then he shouldn't have slept with her full stop. To say it was because he thought Rachel was sleeping with someone else, it basically just makes it revenge-cheating, doesn't justify it. You either end the relationship and move on or you try and make it work, and sleeping with someone else isn't trying to make it work.