That wasn't very nice of him, was it? Take advantage of your feelings for him to get you to have sex with him? I doubt he could trick himself in to actually believing that you would be fine with being friends with benefits.
It's hard to resist the physical pull from a longtime partner. Maybe you can find a way not to see him in private again?
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17