While his mother watched
This happened when I was fifteen, I used to date a boy. One day he invited me home and we started fooling around and I told him to take it easy since his mother is home but he told me he is very horny right now and want to do it (I wasn't a virgin then) but I said no and I would give him a blowjob instead.
He agreed and took his pants down and I started sucking, when I am at it I noticed his mother peeking from the door crack. We left it open so we could hear her coming. I was about to stop but to my surprise she just stood there watching me give her son a blowjob. I got wet and tingly between my thighs and decided to give her a good show.
I adjusted my position so she could see my ass and pussy very well, and I lifted my skirt and I started playing with my already wet pussy. I wasn't wearing any underwear that day. I noticed her reaching down her shorts and I guessed she was masturbating which turned me even more.
After a few minutes I had a mind blowing orgasm while my boyfriend dumped his load in my mouth. I didn't swallow as I do sometimes, it was amazing. I sat on the ground to catch my breath and my boyfriend's mom was gone. I didn't know if she had cum too or not but I did find out later but that's another story.