Plenty of times.

Some were just that, where a roomie was in the room and that was all. Living room sex, or the door to the bedroom was non existent and was right across from the couch, that kind of thing.

Others were where the roomie was specifically enjoying the show because the women I was with tended towards exhibitionism since I was pretty deep into the local scene for group activity.

A few were more organized shows for a roommate's enjoyment, during a span where I had a roommate that was very into watching. One of those times was with a girlfriend of hers that was into guys too, and we got along well enough to perform for my roomie.

Obviously, those weren't casual roommates, or short term acquaintances.

For a while I lived with a friend that was gay, and we didn't have much space, being young and broke at the time. Since I also didn't have any issues with seeing men go at it (though, alas, I also wasn't turned on by it), we tended to only bring home partners that were okay with being seen and not enjoyed lol. There were also several times where we'd both have someone over.

Over the years, most of my roommates were friends first, so I made sure to know what their boundaries were before sharing space. Luckily, being deep into the scene meant that most of my friends were from the scene. So for the ten odd years I had a roommate almost constantly, it was unusual for one to object to sex happening around them at all. Most were not only okay with it, but enjoyed it to some degree.

The few that weren't friends first were really early on, after high school and for a brief span after college while finding the right job. Well, plus a cousin that moved to the area and needed a place while she got situated. That era was also really early into my time in the group scene, so I wasn't often bringing people home. Hadn't built up friendships in those circles yet.

Past the late nineties, I never lived with anyone that wasn't either a partner or quite comfortable with (if not a part of) group sex activities in some aspect.

By the end of the early 2000s, I had been in a very bad car accident, so my group activities were curtailed heavily, as was sex in general tbh. I met and married my wife during that, and we didn't need a roommate financially, so the only times someone stayed with us long term, they were friends or relatives. That's the way things still are, though both of us have "retired" from non monogamy, and don't have any desire to show off. So even if someone did live with us, we wouldn't be doing anything significant around them.

But the decade of the nineties, roommates came and went fairly often, as did locations. Often at the same time. It wasn't anything meant to be long term, though a few people were roomies a handful of times each.

It was a lot of fun.

Well, looking at proper swapping as opposed to group scenes and orgies, it's really about the shared experience. Monogamy is great, but there are some things that you can't do, and can only share vicariously. Being able to more viscerally see and hear a partner being fucked, or otherwise engaging in sex, you get a fuller perspective of how they respond, what they're feeling.

Sure, when you're with your partner, you have some ability to see what they're experiencing, but it's different.

Plus there's the aspect of being able to interact with the member of the other couple you're with. Being able to share the experience of watching your partners do their thing enriches your own sex. Just the experience of looking over at your partners, then looking back and sharing a smile or a laugh, or the enhancement of pleasure is such a wonderful moment of human connection.

Even taking turns, where one pairing has sex while the other watches, and then the second pairing uses the arousal from that to lead into sex, which often feeds back to the initial pairing so they go again is a very powerful thing. If you pick the right couple, it deepens not only your relationship, but the friendship between couples. Which may not seem hot on the surface, but the best sex happens as partners learn each other. So an extended friendship can make the sex amazing between everyone involved.

The camaraderie of it is nice as well, though that's not the hot part. Being able to congratulate friends on how well they've engaged with your partner is lovely.

And, like you said, watching a partner fuck is better than porn with them. It's immediate and real in a way that recordings can't match.

You know the difference between kinky and perverted, right?

Kinky is tickling your ass with a feather. Perverted is using the whole bird

Everything has a risk in terms of bacterial/microbial transfer. That's why you never go ass to mouth, or ass to vagina; the risk is high enough to be prohibitive.

Now, vaginas are just as microbially rich as any other part of the body, so the risk of something transferring and causing problems is definitely not zero.

Luckily, most vaginas are going to have similar species living in and around them. It's all about balance. That's how yeast infections and bacterial vaginosis happen. Something throws the balance off. It can be something as simple as a change in diet or as direct as a pathogen breaching the defenses.

But, it can be triggered by sex. Your dick doesn't have the same proportions of microbes, and the exact strains of each are going to be at least a little different. It's why a new partner can sometimes cause trouble even when nothing else is involved.

With sex, the microbes on the penetrating object are going much deeper than they'd ever get on their own. So the natural ability of the vagina to clean itself out is partially bypassed.

That's where your question comes into play. If you go from one vagina to another, you will be carrying samples of their biomes between them. Assuming no STD involvement, the risk comes down to yeast, staph, strep, and trichomonas (though that is not necessarily going to be a standard part of the biome). Yeast in particular is on all of us, all the time. You can't avoid it.

This means that if the strains of the three people involved are different enough, or their proportions are off enough, then it's fairly likely that one or more of the group will end up with their personal biome off balance. When that happens, one of the species is going to outgrow the others, leading to symptoms.

That risk can't be totally eliminated, even with bathing in between. That would make the risk absurdly low, but still non zero.

The good thing is that since vaginal secretions are going to be balanced for the person producing them, chances are also good that it won't cause any problems. It'll give a bit of cushion for the new body to adapt and rebalance.

Back in my days of doing group scenes and outright orgies, some women would just have yeast medications ready to go. It wasn't like it would happen all the time, but it is part of the hassle of sharing fluids.

Barring a pathogen that's more aggressive, or that doesn't have good treatments, you're looking at mild annoyance rather than danger. Assuming everyone gets tested for the dangerous stuff, it isn't a major concern.

Also, pics or it didn't happen (I'm 100% joking with that, unless that's part of your fun)

Back in the day?

People still do that, though not in public.