Well, I would gain more pleasure from doing the right thing in this case. Cheating, and hurting the feelings of a girlfriend who trusted me to keep a promise would cause me MORE pain than the momentary physical happiness was worth. Even if the girlfriend didn't find out, I would feel anguish because of the betrayal, and I would feel like less of a man because I would have to lie in order to keep a girlfriend. Of course, considering that I don't commit myself to just one woman anymore, and considering that I am honest about this with any girl I get involved with, I don't have this problem. The girls I sleep with know that I sleep with other girls. Some encourage it, a few have joined in. Not every girl is into this of course, but some people are too possessive for an open relationship. Those people should stick to monogamy, and then try to hide under the bed when desire comes knocking.