I've crossed the line, how do i undo it....?

A few years ago, my brother-in-law's friend admitted he had a crush on me, only thing is, he's married. He's been unhappily married for 4 years now/no kids...for a long time i wonder why he wouldnt get a divorce....I knew his situation for a while now but i wasnt interested but remained platonic friends.....to make a long story short, just recently (6 months) i started developing an emotional attachment towards him and we became physical (not ual) for one night and now i feel guilty. I understand any comments will be judgemental and I have this conscience but I dont know how to retract what I did. I dont see him often, maybe once a year but I dont want things to be awkward since he is one of my BIL's best friends and he could possibly be the godfather to my nephew. Should i apologize for crossing the line and leave at that? or should i ignore it and pretend nothing ever happened?