WOW, that is so dam HOTT!! My wife and I once spent the night at a friend's apartment and I pretended to be arelax while she seduced him and fucked him with me on the same bed. I wasn't supposed to know they were doing it. We agreed on this so I would not have to be embarrassed in front of the guy after that day. He just thought my wife was being unfaithful without my consent. What he didn't know didn't hurt, and hell he sure took advantage of the situation and fucked her brains out while I pretended to relax through it.
I can understand such movies being a turn-on...in real life it sure is!