I'm on a few dating websites. My profile on all of them states very clearly that if you're married or otherwise involved, don't bother contacting me. I have had men either not read that portion of my profile or think that they're so amazing (or I'm so gullible) that they will be the ones to change my mind about that. I try to be as polite as possible. "No thank you. Good luck. Hope you find what you're looking for." However, most of the time I want to all-caps their asses and virtually yell "FIX YOUR SHIT OR GET OUT!!"
My most recent two encounters tried to garner some sympathy. "Things aren't good at home." "I haven't had sex in two years." And that's my concern, HOW?? Sorry, boys. You get zero sympathy from me. None. Staying in a bad relationship is a choice. I'm not saying it's an easy one. But nevertheless it is a choice. And as a point of full disclosure here, I confess to having had a past relationship with a married man. It wasn't fun. It wasn't easy. It wasn't worth a few moments of seeing him to go through all the rest of it. Guilt. Hurt. Resentment. And the biggest argument against it: you, as the "other woman" will never be first for him. You will never be his priority. You will never be the one he chooses. And if by some miracle he does get a divorce and decides to be with you, you are now involved with a man who doesn't try to fix what's wrong in a relationship, but instead seeks fulfillment elsewhere.
My conclusion: A no win situation if what you're searching for is a healthy, fulfilled, full-time relationship.