No, I don't know what/how a man is thinking.

But I do know when he's lying to me. So I can tell you that not every heterosexual married man (or man in a committed relationship) thinks about having sex with other women.

What bothers me about this attitude is that it's one of the very few times when people who argue the way you have think it's ok to accept a stereotype as pure fact for everyone.

Not everyone is the same, not everyone thinks the same, not everyone has the same urges. And some people hve managed to let their principles guide them to the extent that they don't even see other women in a sexual way at all. It can be done, and to assume otherwise does a disservice to men.

And, really, you need to remember what I said in other posts below: imo, cheating in the mind constitutes more than just thinking "she'd be good in bed". It's thinking about actually doing the act with that woman in a specific way, not some abstract concept of sexual attraction.