I haven't thought about this in a long time, but reading something on reddit today triggered this memory.

This happened when I was much younger. When I was 22 or 23, I used to hang out a lot with my at-the-time girlfriend and her younger sister, who was either a freshman or sophomore in college when this happened. Let's call her B.

B had always seemed to be fond of me. A little giggly around me and a bit nervous. But she was my gf's sister and in high school when I met her, so I never thought much of it. I also rarely hung out with B alone, but there were a few things we had in common that her sister didn't really care for, including going to the gym.

This one time, I went to the gym with B, and there happened to be no one else there. We were working out for a while, and I had spotted her a few times where I was actively touching her all over and was pretty close to her. I guess this was turning her on or B didn't want to let the opportunity slip away, because then she asked me to check out how strong her abs had gotten. This was a bit of a joke between us. So I poked her in the stomach, and just said "nice." And I thought that was it.

But she said, "No, really, feel." And she grabbed my wrist and pressed the whole palm of my hand on her abs, and moved it up and down to feel her stomach. I'm not going to lie, I was already turned on. She was a slim little blonde thing, and she had a developing 6-pack.

And then B moved my hand higher so I was now cupping her boob. And there I was, frozen, looking in B's eyes with what must have been shock, her holding my hand to her boob, and I just started massaging her nipple with my thumb. Instinct took over, and I moved in to kiss her, and she closed her eyes to get ready.

And then my better judgment took over. And I backed off and said, "Yeah, your 6-pack is really coming along." And just moved away from her. I've done some shit in my life I'm not proud of. But I wasn't about to hook up with my girlfriend's little sister.

We never talked about it. Nothing like that ever passed again between us. Her sister never found out.