This experience happened to me a few years ago and it ended up being a couple that I got to 'join' about once a month over a 6 month period. They were pretty hardcore swingers and I think I learnt some good skills (particularly from the husband on pleasuring women).
This is what I wrote at the time just after the first time we met.
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Last Friday night I had a threesome with a couple that I met online. A friend of mine recommended this couple as good fun. I spoke to them online before I arrived here and they seemed really keen and also rather experienced.
Friday night came along, I met them at a bar in town after dinner. Lots of photos were exchanged beforehand and they looked like their photos. I told them I was bi and they said that the husband is 'open minded'. After just one drink we were getting along fine and the wife asked if maybe we should head back to their place for another drink. It sounded good to me as I was getting a hardon in my jeans and black speedo already.
Their house was only a 5 minute walk from the bar.
When we walked in the husband went to the kitchen to make some drinks and the wife pushed me down onto the couch and we started making out. Our lips (and tongues) were locked until the husband came back with our drinks.
At this point the husband sat on the couch next to me and his wife got off my lap.
"Well Dave - you told us that you like bikinis and speedos. How about you guys get down to your speedos and I'll go put on a bikini..."
While the wife left the husband and I did what we were told, we stripped down and it turns out we were both wearing a black speedo under our jeans (I had told them I'd be wearing speedos under my jeans before hand).
I was hard as a rock from making out with the wife and the husband was slowly getting there. It was pretty hot sitting on the couch next to a married guy, both in speedos, rubbing our cocks through the lycra waiting for his wife to join us.
So the husband and I are sitting on the couch both wearing black speedos. I'm hard as a rock from making out with the wife and we are both sitting there rubbing our cocks through our speedos. It doesn't take long until the wife returns. All she is wearing is a white bikini. She is petite, nice medium sized titties, shaved all over and brunette.