Mark is a very strong person, dealing with a shitty situation the best way he can. He's very mature and pragmatic.
When I was away for uni, Magpie spent an awful lot of time with a friend of ours (she's now dating my brother, but was single at the time), and even though I KNEW nothing was going on, that that was pretty much impossible, I still felt like I wanted to be there; I wanted it to be me who was treated to Doctor Who and Indian take-away on lazy, hungover Sundays. I was jealous, but not because I was worried about being cheated on, just because I wished it could have been me. And I think that't what Mark is feeling.