Twice and both happened during my uni time.

One was after a new years party at a friend of a friend’s place who I didn’t know prior. It was late and I was about to leave with my buddy when his girlfriend arranged with the host for me to crash on her couch instead. It was easier for me that way so I agreed. Later I learned my bud and gf wanted to have sex and she wanted me gone haha. The host gives me a blanket and pillow and I prepare to sleep wearing only my boxer briefs. We got along super well all night before but since I knew she just left a long term relationship, I did not make any moves. She comes over in her pajamas.

her: are you tired yet?

me: no I had too much coke.

her: same. … you know what usually helps me fall asleep?

me: what?

her: a good orgasm.

me: he, uhm, same.

her: well then undresses

We slept well afterwards. I later learned this was also part of the plan of my buddies gf. It did stay a one night stand however.

The other was during a party at our flatshare. We were 6 flatmates and during a party we would always lock one room to store clothes, backpacks and valuable stuff. The parties could get pretty big so it was safer for everyone.

When it was my turn to have my room as locker, a girl I met a few days back followed my invite to the party. We went into my room, I took her jacket. She wore a stunning short dress under it, and clearly no bra. I must have stared noticeably but quickly went to put the jacket away. I here the door being locked and turn around to find her dress and panties on the ground next to her.

She later said my look at her in that dress gave her the confidence to pull that move. We dated for a few weeks but did not get together.