The main purpose of dating someone is to build a future with them.
The chance of ruining his marriage isn’t’ worth your loneliness. You may think it’s terrible based on the fact that he’s out with you, but the truth is, many men have it great at home and love their wife, they’re just out looking for fun.
Frankly, your conscience is as stupid as your heart and knows not the sacrifice it’s asking you to make. Let’s face it, nothing says “I hate me” more than to willingly accept the second-class stepchild status you’ll be relegated to for the entirety of your relationship with this man. Better to enjoy the short end of the stick than have no stick at all, even if it means destroying a family a tiny little bit.