Another thing to consider: Even if he changes his cheating ways for you, do you really want to be that woman -- the one who wreaked havoc on his marriage in the first place? Speaking of his marriage, even if you're convinced that he's going to leave her, don't delude yourself into thinking that this is a simple process. Breaking up is hard to do. Think of the end of your last relationship, add several years or a lifetime, throw in a couple of "'till death do us parts," multiply by a couple of kids and prepare to divide all of your material possessions. That's what's on your guy's plate.
Still Lonely ... After All These Years
What you have when you love a married man is fleeting, a wave that crashes over you (perhaps knocking you down) then vanishes into the ocean. That's it. You can never put his name down on an application form under "Who to call in an emergency". You can never count on him for weekends or holidays. You can never "work on the relationship" or expect him to make compromises or even show up on your arm at an office party.
Your married man might never bore you or irritate you with his weird little habits. He's never around long enough to make you feel comfortable enough in his presence to find him wanting. But he can never really make you feel good about yourself. If you did, you'd want a man who considers you his partner, not his paramour.
The Real World
Real relationships aren't about diamonds and caviar, tempestuous sex, illicit thrills. They're cotton, not silk. They're morning breath and arguing about whose turn it is to clean the toilet. They're also about having someone curl into your body the whole night long and to listen yet again to your diatribe against your unfeeling boss.