Dear LIB readers: I had a fling with a guy and he's now blacklisted me

From a female LIB reader

Have I made the biggest mistake of my life? Did I do something so terrible? I met this guy at my work-out place and found him really attractive. I've never found myself making the first move but in this case it was almost uncontrollable. I got his no and called him and we met up, had drinks and I couldn't control myself either and it all went down. He called me up for a second date and we had sex again. We both agreed it won't happen again since he is in a relationship and wouldn't want to mess that up. I see him everyday at the gym and we just greet passively. I just realized he blacklisted my number on his phone. That hurts. It's not like I was stalking him. Please did I make such a blunder to follow my heart for the first time ever? I'm just really confused here. Did I do something that's unacceptable?