You're married and have no intention of leaving your marriage. Hopefully during all the years you have known this man you have believed yourself to be "friends" with this man rather than "in a relationship"?
You do not want to "take things to the same level he is asking" - what has the level been over the years- and what exactly level is he requesting? Is his current request sudden and a surprise to you?
Without understanding the background, I am at a loss as to how after so many years this male friend, who has his own partner would suddenly ask a married woman who he has known for years that he loves her and wants to "take things further"?? Thus it's difficult to answer your question/s one way or another. Apart from the fact of course taking things to another level would surely be hurtful for your husband and his long term partner.
What do YOU think is the reason for your male friend's sudden change Jen67patte? Plus is his long term partner a joint friend and do you and your husband socialise with them as a couple together? I just don't understand the history and dynamics of your friendship with this man or how that sits with your husband and his long term partner. Sorry- your post raises more questions than answers for me I'm afraid...