Would you

You always know it will wreck relationships. You never not know if it will or not. It always will. But maybe you mean if it's possible to do it and not get caught? That's a better question. And the answer is yes. You can do it and not get caught. You can do it and it doesn't have to cross over into your life and ruin what either couple has going on. It can be compartmentalized and separate.

Yes

I'm a female. Never been married, Never dated, Never had a boyfriend, I'm frumpy looking, I'm a BBW, only 5 ft. 2 in. weigh 322#, have 44 FF boobs, big belly, huge butt, and super thick thighs. Not even a lesbian, can't even get a date with another girl.
Anyway, my best friend is a chubby girl, at least she's gorgeous. Her husband is a skinny little twit. But he loves big women.
For his 25th birthday, my friend asked me if I'd be willing to have an FFM three some. Knowing I've never been with a man. I took my own virginity with a dildo at the age of 21, so I agreed.
The evening came, we got together for dinner and drinks at there place. We all retired to the bedroom. Things started off slow. But pick up speed rather quickly. We did some girl on girl, while I was eating her pussy. Her husband fucked me. He wasn't even as big as my dildo, but dam did he feel good. Especially when he cum inside of me. He fucked me three times that evening. Which made my friend, is wife a little upset.
Her and I talked the next morning, and she said she was okay with everything. Which was all good u til two months later when I found out I was pregnant. I didn't tell my friend until I was about 6.5 months along. Because my belly was so big already I didn't actually start show till then. My friend was very upset, we didn't talk again till after the baby was born. We still talk now and then, but we're not friends like we use too be.

The only situation you have is retardation.

Off limits in my book. I suspect my ex best friend did . My wife swears up and down no but I'm 95 percent sure. I have wish bad I'll on both if they hooked up . She got cancer and he homless and walk like a hunchback now.

My wife and I had sex with a good friend couple, it was fun. My wife and I were 42 at the time, she’s been best friends with a girl since high school and her husband and her one night got drunk with us at a party. We swapped partners dancing, it led to touching and kissing, and sex with each other’s partners. It was supposed to be a one time thing, but it wasn’t. Her friend and I started hooking up regularly for quick sex, and we found out my wife and her husband were doing the same. It was some good time for about two months before the ladies put a stop to all the fun.

My husband and I swapped for the first time during the covid lockdown with our older neighbors. My husband was 29 & I was 26 at the time. Our neighbors are early & mid 50s, have professional jobs and in great shape for their age.
We were invited over for a secret cook out by their pool. The alcohol was flowing and everyone was flirting. They broke the ice and told us they were swingers and their summer plans got canceled with the group they vacation with. They asked if we'd be interested in swapping. They assured us they'd never try to break up our marriage and they had plenty of condoms available if we agreed.
When I first got married 4 years prior, I'd never thought I'd do something like this but at the moment it just seemed thrilling and fun. We agreed and we are so glad we did!! We still swap with them a few times a year and we've also went on vacation with them with their friends.

Six years ago, I had sex with my friend's husband that we'd known for 20+ years. It was a one time fling. We've kept it secret and thankfully things have never gotten weird or awkward.
It was the first year after all our kids had moved out so we decided to rent a beach house.
My friend went to a play solo. I stayed at the house to enjoy the beach. My husband and her husband came back from golfing with a couple bottles of bourbon. My husband had too much and we helped him to the bed.
I'd had a few glasses of wine and feeling buzzed. We've always flirted when we were alone but this time things got heated. I sat on the railing and he stood between me.
The next thing I know, his penis was out, my swim bottoms were pulled to the side and he was going in and out of me.

Personally, no unless invited by both. Not worth a friendship over a piece of ass! Just my 2 cents. Let fantasies be fantasies. Unless he's a shit friend, then whatever.