I deeply symapthize with you. I have been there and I have had the same feeling of wanting to be with this person forever. I could humbly feed on the crumbs that fell from the married couple's table, knowing that I was not really in a position to make great claims, I went on stolen moments, interrupted telefone calls and a lot of loneliness, nights, wekends, holidays. I thought time will make my love so apparent and so overwhelmingly convincing that it will just play out - somehow. I wished him t find happiness, not to poses him, But yes I hoped that happiness would be with me.
But fact is, almost all men who haven't made up their minds until at least half a year into the affair, are always less likeley to leave their marriages as time goes by. Reasons always sound compassionate and considerate (kids, their wife would be broken etc.), but they are not compassionate towards you.
You are the one who gets to stand as the lonely observer on holidays, valentine's day, christmas, family vacation trips. You are the outsider and she is blooming in his presence. She is the one everybody will feel for when things break apart and you are the one who will be blamend. If he hasn't told his wife yet, just wait for the momennt she finds out. It will be a crucial moment: Is he willing to stand by you or will he cast you aside in a minute, telling you things would have to change from now on and he just can't bring himself to leave his kids and hurt his wife. He isn't that kind of a man.
I think the only chance is that you set him a dead line. If you really feel he is the man you want to be with, tell him to give you certainty and take definite steps within - lets say - half a year. If he doesn't he'll always have an excuse. You will always feel more hopeless, the status as it is, the longer you accept it, becomes almost more comfortable for him and - one big thing: Your relationship honeymoon is expiring day by day and getting more ordinary, until he sees no difference between what he's got and what he's about to have.
But when you set an ultimatum you have to be ready to take two extremes: either immediate withdrawal. Or the opposite, he really changes his life for you. In that case be prepared for anger by his wife, rejection from his friends, grudges from him, at least tons of grief for a while, fear he might run off one day to return to his family and, who knows, when everything is over, he might find another woman, a fresh and innocent person, one who wasn't the one who made him break up with his life.
Statistics is almost 100 % against your hopes. It sure happens, but it happens very rarely that happy relationships come out of betrayal. I am not prude. I just know.