This is actually a mff story.
I had a ten year relationship with J, she was still a teen when we met but was far more sexually experienced than me having lost her cherry, voluntarily, at thirteen.
About four years into the relationship she turned up at the swimming pool I managed one evening with her 13 yr old cousin, let's call her Dawn, just as I was hosing the changing rooms out. J suggested we went for a swim, I pointed out that we normally did that naked, and Dawn replied she didn't mind but had brought her black one piece costume.
J and Dawn went into the ladies changing room as I finished off the male, then I went pool side and stripped off before diving in.
After a few moments the girls appeared, J naked, Dawn wearing a costume that looked like a leotard. They got in the water and J swam across to me.
It wasn't unknown that we would end up making out in the pool, but I certainly wasn't expecting her to start teasing me like she normally did, especially not with Dawn swimming close by.
'Oh she wants to see' was J's response to my muttered comments about the situation.
Okay, maybe its a bit kinky for some people, but I asked Dawn and she said yes, so we got some of the kids swim mats and put them on the poolside, and then started to show Dawn the sort of things we did.
I'm talking oral, both giving and receiving, breast play, fingering and masturbation.
Whilst I was fingering J that she moaned 'Look at Dawn'.
Dawn was sat on the front row of seats, her legs spread, costume pulled to the side, and her fingers flicking over her bald clit. Her nipples were showing through her costume like coat pegs.
J had put her own hand up to caress her own clit, and I could feel her cunt clamping on my fingers in orgasm.
I can only assume it was her moans that tipped Dawn over the edge, she started to yelp and her fingers flashed faster, before a loud cry and a sudden stop.
She'd realised we were both watching, started crying and ran into the changing room.
J took a few seconds to relax, then moved forward and went after her.
I knelt there, my cock stiff. Maybe not a good idea to follow them in in my present state...
So I gave it a couple of minutes, it flopped rather than totally collapsed, then wandered in to see what was happening.
J and Dawn were sat opposite each other, both naked, masturbating as they watched each other. Whilst J was fingering herself with one hand and clit rubbing with the other, Dawn was just clit rubbing and using her second hand to squeeze her own nipples.
They ignored me as I immediately stood to attention and started to wank.
I don't think Dawn looked at me until I walked closer to J and shot my load on her face and tits, but within seconds they were both orgasming again.
If this was an invented story the threeway relationship would continue, but after that we barely saw Dawn. A few months later we moved, and three years later we split up.
But that evening still remains probably my most erotic experience so far.