1. Sharing my then-wife with her ex-bf. I fucked her doggy-style while he lay underneath her and licked her clit and pussy lips.

2. Being seduced by an older man when I was in my late teens. he had a home sauna and gave me a long slow massage which ended when I came in his mouth.

Many others, but those are two stand-outs.

My ex-wife was bi, and for about a year we shared a girlfriend. One night my ex was going to a strip club (traditional, with female strippers) with several bi friends, for lap dances. Our GF and I weren't invited, so we went out to dinner at a nice place by the beach, an upstairs restaurant with sunset views and a bar downstairs. We enjoyed our meal, during which I discovered that she had no panties on under her skirt. After, we crossed the street to the beach side, and started kissing. Some folks from the bar started to cheer us on, so I turned her around and, with her back to them, lifted her skirt and cupped her bare ass in my hands, eliciting louder cheers from the bar.

From there we drove to the motel that was our prearranged meeting place, but I was almost out of gas. Before checking in, we stopped at the gas station next to the motel. This was before every station had self-pay at the pumps, so I had to go inside and give the attendant a $20. He kept glancing out the front window. After I paid and went out, I saw what he was looking at--our GF had her door open and was sitting with her legs spread wide, masturbating in full view of the attendant. She kept it up while I filled the tank.

Then we went next door, checked in (our 2nd floor room, coincidentally, looked out on the gas station let, as did several of the other motel rooms, and started fucking. I had her up tits up against the window, fucking her from behind, when my then-wife pulled into the lot downstairs. She saw what room we were in by looking up and seeing our GF being fucked against the glass, so hurried upstairs. For the next several hours, we fucked in every combination and I spilled several loads in pussies and mouths and on tits. Hottest night of my life.

Last summer my 22 yo gf and me went on a hike around the mountains, finding a little lake (like 10 meters wide) and some small waterfalls.
We were so horny so started swimming naked and making out till we went to the shore and I started eating and fingering both her pussy and asshole. After a nice time between her legs we switched sides and she started blowing me softly but steadily. Like 5 mins into the blowjob, a random couple emerged from the forest into the opposite shore. Being a little inhibited we stopped and waved hi to the newcomers while moving a bit far from the lake out of sheer shyness since they had clearly see us naked and with my cock 4 inches down her throat. (I should add that we're a pretty vanilla couple, we love sex, but nothing too kinky, outdoor nudist in random unpopulated places like that was the kinkier we had ever get).
The other couple waived hi back, they act totally normal. He was 40s yo, slim, tall and looked a bit hippyish, she was 30s, dark skin, small perky boobs and also that hippy look. To our surprise they got fully naked and started swimming around the lake to get the heat out. Then they swam back to the opposite shore and started to make out fully nude. Things easily spiced up and he fingered her while she jerked him. I looked at my gf and she was observing them with the same arousal and schock that I had so we started making out too and heating her up by rubbing her clit and having my thumb playing down her asshole. She got really wet and lay down to our previous position, starting (or re-starting) to blow my cock off, with a slow but steady pace, always willing to go deeper in the next blow, and not affraid to be sloppy. The hippy couple also decided to raise the bar and, while he was lying in his back, she sitted down her average sized but quite wide cock and started riding him cowboy style, moving her petite ass.
That next little while was probably my best real sex-life experience, being in the middle of the wild, naked, getting a top blowjob from my cute gf while a couple fucks openly just 10 meters from us. Our four heads spinning once in a while to check how the other couple was doing, sometimes a quick sight, sometimes a clear stare, even direct eye contact. Both our moans were easily audible from the other shore. I've never been shy to moan loudly if I enjoyed a blowjob or a fuck, and apparently so was the other guy, who was almost louder than her. Can't blame him, according to her waist and ass moves I'm sure she was doing a great job. The rest of the sounds, I mean besides the moans and heavy breathing, were a sight to be enjoyed as well, from the birds singing above us, to the small waterfalls leaking next to us, to the clapping sound of our new friends thights while she rithmically rode him, or the drooly, slobbery sounds that my gf's mouth does when trying to choke itself with my dick.
I'm not sure how long that was, maybe 10/20 mins but was glorious. According to his moans he eventually came, they spent some minutes just hugging each other, packed theit stuff and fully naked, just with their trekking shoes, moved to another pound like 200 meters down the river. They just waived by, not a single word was said between the two couples. My gf was still blowing me during the whole process, she's got great endurance, but I finally nutted almost as the other guys left, creaming down her throat and mouth and inmediatly started making out after that.
We took another bath but quickly packed our stuff and went home, where we fucked for hours just for how horny we had get.
Up to this day we still bring that story up, sometimes feeling bad for not going further (I'll let your imagination to know what that meant), sometimes feeling happy that we had a nice hot story but didn't push it to somewhere that we as a couple wouldn't have felt comfortable.

TL-DR: While being in a almost isolated lake in the wild my gf gave me a BJ while another totally unknown couple fucked on the other shore.
Sorry for my English mistakes, not my first language.

Probably an MFMF foursome with a Canadian couple we knew back in 2014

Been there, done that.

Our State Farm insurance agent, Paul, had a thing for my wife. In exchange for a bundled deal on our insurance policies, I arranged to have him fuck my wife in his office while I watched. Four times a year when different policies came due...homeowners, life, auto (2x). Saved about $600 annually doing this!

Fucking two marrried women together and watching them play with each other, the cumming in both pussies - 3 spurts in one and then hold and enter the other to finish

Wow two married women playing with each other
Very horny

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I'm amazed, as always, how many guys respond to a topic in the 'For Her' board.

LOL it comes up in "Today's Active Topics", I don't look to see which board. Anyway my story is tame compared to all these Casanovas. I got blown while driving a '95 Mustang GT with a manual transmission. Which, if you think about it, required a lot of acrobatics from both of us.