It's not really that hard to understand; even the biggest jerks tend to be emotionally attached to somebody they've been with for a while. They didn't get into a committed relationship for giggles, even if they did it for the wrong reasons, or have trouble sticking by the whole "committed" part. In the end, that tends to win out.
This doesn't mean that a cheating partner should be forgiven: that's up to the injured party. It does mean, though, that sleeping with the married is a loser's game; even if he does leave the wife...consider that makes the "other woman" the one he can cheat on now.
*sigh* maybe because he loves her and is only using you for sex for whatever reason that is obviously none of your business if he hasn't told you yet. maybe despite the fact that he's a cheater he believes in the "for better or for worse" promise even if he boinked out on one of the other ones. maybe he has children with her and doesn't want to leave that legacy for them, and thinks cheating on their mother isn't "so bad" as divorcing her for someone else. maybe he doesn't want a woman who is irresponsible enough to cheat with him raising his children every other weekend. maybe he can't afford a divorce because his life with his mistress is only about sex here and there one or two hours a month, whereas his property division, assets, and custody issues just make divorce too expensive and complicated and he doesn't care enough about you to go through all of that. maybe he doesn't have a job and would end up in jail for not paying child support if he left her and his cheap sex on occasion would now turn into a situation that would ruin his life, his credit, and his record. maybe she told him once if she ever caught him cheating she would cut off his balls and hang them up in the square for the world to see and he believes her. not saying the guy is a prince or anything, but really, could be anything, every guy is different.
Maybe he just don't love you enough to figure out ways to face the outcome to leave his wife.
Because losing half a house, their kids respect, their mutual friendships, and the in-laws civility is inconvenient to them.
People who live double lives for a meager sense of self-worth don't love anyone, let alone themselves.