True story:

I was in my early 20s. I'd been hanging out with this "lesbian" chick for a while. I knew her since childhood. She gets a thing for me despite being a lesbian. We go to her place. Get naked. Get on bed. Start fucking around. Then her 300lbs bi-sexual girlfriend walks in and wants me to fuck her too. Starts touch my dick, etc. Instantly limp. I ended up leaving.