Regrettable sex I had.

A few weeks back, I went for a date with a guy who is in his mid 30s.It went well and slowly we felt a connection and so ended up having sex.We enjoyed each other's company and so we continued the fling for a while until one day he asked me if I don't remember him.I was confused and said I didn't.He smirked and confessed that he used to be one of my high school students back when I was a new recruit in mid 2000s.It came as a shock for me and I stopped everything with him from that day.What's infuriating me is that guy knew that all those time he was with me and yet decided to hide it.I regret every moment of it now.