It's not that an uncommon experience I think what you are going through. My brother is going through the exact same thing at the moment. He is single, but has developed an attraction to a married woman and she is really keen to move ahead with him, but he is uncomfortable with the secrecy etc and so is slowing things down and encouraging her to talk to her husband. They are virtually separated but living under the one roof. She comes from a strong religious family and it is very difficult for her to talk to her husband and family, but ideally this would be best...
But it is not a black and white situation.
Does the guy who you are interested in want to be with his wife? Are they married but essentially separated? Does he actually love both of you?
The healthiest scenario for all concerned is openness and honesty. But that is not always possible for all concerned for many different reasons. Only you know your situation in all its complexities - trust your gut instinct and let it lead you...