I'm pleasantly surprised to see so many guys who would NOT sleep with someone they knew was in a committed relationship. It seems so many gay/straight/bi men DO, from what you see in the media, etc.
My take on this is this: Anybody who has to sleep around has big issues (immaturity, insecurity, incompleteness, lack of self-esteem, etc.). They may couch their issues in other popular jargon: "Men are naturally on the hunt/prowl, sowing their seed subconcsiouly for the perpetuation of the species. It's only recreational sex. As long as nobody gets hurt . . ., etc., etc."
I believe the nature of the sexual act(s) is intimacy in the closest possible way. When that intimacy gets diluted with multiple sexual partners, you lose something of yourself, not to mention the other person's self. That is the whole reason for making a relationship committed: to perpetuate the bond of intimacy, through thick and thin. "Open" relationships therefore, are not committed, according to my view. They are immature, insecure, incomplete, and lack self-esteem. A mature person has the ability and energy to make and keep a commitment.
MANY people today do not have that maturity, for any number of reasons. They have never had it as a model (re: children out of divorced parents). I feel very sad for them. They seem incapable of living towards others, but mainly towards themselves. They will never know the fullness of being really human, which comes from living more and more for others.