I do not recall whose idea it was but somehow we ended up making out with the other spouse. Eventually we got into the house and we each had sex with the opposite spouse at the same time, which the husband’s came at the same time. After some other foreplay activity, we then continued in the doggie position in front of a big mirror in the living room. I recall we were side by side looking at the mirror to watch us together as my larger breasts and our friend’s breasts were bouncing away. Then we pretty much were exhausted. The next day it was a very embarrassing memory for me, at least. None of us ever discussed it or did it again, even though we remained friends.
They got divorced about 3 years later but my husband and I are still married for 40 years now. I think this was in the “swinger” era, so I’m thinking that is why it happened. When we were in our 20s we had another couple over for dinner and then ended up in the spa. We added bubbles to the spa and with some heavy drinking, our bathing suits came off although still not showing much behind the bubbles.