Not sure there are any bad reasons for sharing such a wonderful experience with someone. Even if you will never be together again.
I do regret that I was not more cautious with my reputation in school. But, the sex I am not sorry at all. Not even once.
Worst for me was when I kind of just went with it because my friend was with the other guy. Me and my friend met 2 dudes way back on a trip out west for competition. They had cars so we went along with them, we both had boyfriends and I never even expected to kiss far less have sex. We just kept kind of going further and then we were back at their place and she went with hers and I thought, shit you're here now so might as well. Later she said she was just doing it because I was and she was following me. We agreed not to tell anyone and that was it. sex wasn't bad to be fair, he was ok lol