The fact is that, if a woman's had a lot of casual sex, she's developed a taste for variety and you have no hope of being her ideal man. When you're in bed with her, she's wishing you had Biff's abs, Jethro's penis girth, and the "roughness" of that club promoter whose name she's forgotten. So she's likely to cheat after a while in search of those traits, and even if she doesn't go off and do it, she's going to be mentally cheating all the time by fantasizing.
I'm a guy of mostly above-average physical traits and skill, but it's humanly impossible for any man to be the best at everything, for every woman. I don't have, for example, a 10-inch dick that's 5 inches wide, and I wouldn't really want one, to be honest; I'm more than happy with what God gave me. But that's what it takes to please a woman who's had 20 partners, because she's taken a lot of huge dicks and is "stretched out".
A woman who's been with too many guys is always going to be missing that one thing that some other guy did or had, and nobody wants to be with a woman who's going to be constantly fantasizing about other men.