But if you choose to look elsewhere for a single man, that is alright to, and let him continue on with his kids and maybe meeting a partner with a similar life situation.
I think without juding that every human being needs company in one point of our life and we look for it but when we look for our happiness we don't see more than that. You should be able to build a family starting for looking a single guy for u star a family without hurting or feeling bad about anything. hope you are fine and in perfect condition of health.
if he had any respect for you or his family or even his future family with you.... if he is not happy in his marriage then he needs to end that first, before building into another entire relationship.
At least then it will better and with better chance to success. you need to decide if you can live the rest of your life being a homewrecker.
Too often we don't commend men for working to keep families together, and he is doing just that.