There's an unhappily married man who I know that I've decided to date.
Seriously? Why would you want to date someone who is married?
I think it would be an interesting thing to do.
It would be an irresponsible thing to do to date someone who is already married. Even if he claims his marriage is unhappy, you only have his word for that. It would be better to find someone who is not already in a relationship.
Well I can't just listen to your advice. I'll do what I want.
You're going to get hurt not trusting my advice, because nine times out of ten the married man stays with his wife in the end anyways. No one is going to leave this affair happier than before they got into it, no one.