(Sorry, this turned into quite a long read! It's my first GW story, and I want to make it good ;)
When I used to work retail, I met and dated a girl who also worked there. We ended up being together for around 4 and a half years. One of her best friends who also worked there, was dating her brother, so we would double date etc, have dinner together, that kinda stuff. I always got on really well with her friend, but not enough to threaten our relationships. Then one year, both of our relationships fell apart around the same time, within a few months of each other.
After that, we all stopped hanging out. Except, I would see my ex's friend out a lot at the weekends. We understood that being seen together might upset our ex's, so we agreed to restrict our friendship a little until things had cooled down, but it didn't work. It only added a danger factor, and that danger factor added excitement. What used to be innocent, quickly turned into lust. Which then turned into a game of teasing and sexting each other. We started to enjoy the danger, and started to really lust for each other, but still respected the feelings of our ex's (the brother and sister).