Thanks for your reply, that's really helpful.
The capital letters say it all! When it comes to conflicted feelings, it can help to think of them as driven by the head (rational) or the heart (emotional). Your heart longs to be with him and sees him as the solution to your loneliness. Your head reminds you that he is married. Your head has been in charge so far but the heart is having a last ditch attempt to get what it wants!
Of course if you go ahead things may work out for you. But the odds are against it. I have seen this pattern many times with many people. It usually plays out like this (be warned this is a rather sorry tale):
At first everything is blissful, all that repressed longing, the danger and secrecy make for a heady emotional and sexual mix. As you are already so close, your feelings will most likely intensify and deepen very quickly. Then he will go home to his wife. No matter that at the outset you thought it would be OK. Now it isn't.
Women in this position become intensely preoccupied with the life which is taking place elsewhere, the legitimate family they are not a part of. On important occasions, they are alone, commitments are broken when plans change. They feel rejected, more alone than when they were single. It becomes clear that they are living in hope of a separation which is not going to happen. They feel trapped as they will never meet anyone else. Hope begins to fade.
Whenever they try and leave, their partner wins them back again through their genuine expressions of love. In the gaps they are more lonely than ever before and very vulnerable to the reminders of intimacy, this eventually leads to a resumption. The head heart conflict becomes more pronounced, the woman more distressed.
Typically there are many attempts at leaving, periods of attempted no contact and reconciliations. It is an intensely sad and distressing process to witness, never mind to suffer. People have described it to me as being akin to an addiction. Eventually it begins to compromise their sense of self and they get out for good, usually via a complete break in the relationship (friendships are rarely possible).
Of course this is not everyone's story. Some have a difficult start as old relationships end and they start new, happy relationships and never look back. Some have relationships and manage to remain emotionally aloof. But the majority seem to go through a remarkably similar process to the one I have described.
His description of a "no fun anymore" marriage suggests that he may be looking for fun, rather than a deeper attachment. In addition it suggests that the marriage is not fundamentally unhappy.
He could be your next relationship, but so could others. I wonder whether you are pursuing other possibilities and if not, why not?
We all need love, but please take care that the ways you seek it do not do you damage.
I'll pause here to see if you have any comments or further questions. I hope you find my answer helpful.