I don't think so. What causes a person to cheat is so complicated, I don't think it can come down to something like the size of his penis.
Studies have shown that the more attractive a man is, the more likely he is to cheat, especially if he's considered to be more attractive than his wife, whereas if the woman is considered more attractive than her husband, their marriage is more likely to be stable. There was an article about it on the Psychology Today website. I suppose having a large penis could work in a similar way as being attractive, although I doubt many women actually seek out mates based on penis size.
http://www.psychologytoday.com/blog...t/200804/why-handsome-men-make-bad-husbands-i
According to this article, I'm screwed! My husband is younger than me and extremely attractive. College-aged women hit on him all the time and I'm in my 30s. But people are more complicated than just the length of his penis and the statistics gathered on marital success, and I think we'll be just fine. I seriously doubt TheBoyfriend would ever cheat on me, and that's because I know that he's a man of very good character who loves me and who is loyal to me.
I was cheated on by a ex-boyfriend who was also extremely attractive, but I think the reason for his cheating had to do with his youth, he was 18, and pressure from his parents to "explore" being a young man. They didn't like me, obviously. They told him that such a young man shouldn't be tied down and committed so young, etc. He did explore, although he didn't break up with me before he did it, and in the end he was very sorry for his actions. He cheated out of social and family pressure to behave the way that other people thought an 18 year old should behave. Did that have anything to do with his penis size? No. His attractiveness made it easier to find interested women, but he was pressured into behaving in a way that was contrary to his actual character, and he's felt guilty about it ever since. We were talking on the phone a year ago, and he apologized to me for cheating on me, just brought it up out of the blue, and I felt sorry for two things: that he still felt so guilty so many years later, and that talking to me made him feel that pain again so that I was a constant reminder of it.
Anyway, I've digressed. My point is simply that the motives for cheating are going to be different for every man, and I don't think having a larger penis gives a man more motivation. I least, I hope so.