Most Helpful Guy
My guess is that it's usually because there's something significant missing in the relationship with his partner. He's not getting what he needs from her - perhaps sexually but more likely emotionally. I think guys are probably more likely to go with another woman if the current one is making him feel badly and the other woman makes him feel good.
If the relationship was decent to begin with and not just based on sex, then I think most guys would stay faithful even if they're not getting as much sex as they want (as long as they're getting a somewhat reasonable amount) as long as the emotional connection is still good and he enjoys being with his partner.
On the other hand, if she's yelling at him frequently or doing other things that make him feel badly, he might be more likely to seek out another woman even if he's reasonably happy with the sex with his current partner.
In other words, I don't think sex is usually the primary reason. I think it's more often emotional rather than physical.