This is a really nice site one of the few that is not particularly judgmental. In response to the last message he sounds like he wants his to have his cake and eat it. Does that sound fair to you, to use and have fun with you for a few months and then go back to his wife. If you can cope with that then put up with him, if not leave him before he leaves you.
I wanted to share a more positive situation with you. I have just started an affair with a married man of 4 years and I was with my partner of 10 years with whom I have a child. We were both desperately unhappy with our current relationships and tentatively agreed to start to see each other and take things really slowly whilst we sorted out our current relationships. It didn't quite happen like that and very quickly we fell in love and then the relationship became physical. I couldn't cope with the guilt and have left my partner and I move out next week into my own place with my son. The man I love has a slightly more complex situation as he works with his wife and holds quite a senior position within the school where the three of us work. He has told her he no longer loves her and is trying to disentangle himself as carefully as he can without wreaking his career.
It's working because we have been completely honest with each other from the start and told each other about all the bad things and talked about the guilt not just creating an idealistic fantasy.
Any one is capable of making a poor choice, particularly when you are younger as both of us did. We are only in our 30s and far to young to sit in loveless relationships waiting for children to grow up an leave home only to be 'dumped' then. I feel awful about the pain this is causing his wife but hopefully she'll find someone new and in the end be happier.
Good luck with what ever choices you make