Cool, I'm not the only one :) It's actually a huge turn on for me to share partners and always has been, though I understand that there is a big, big difference between walking in on it and seeing it unexpectedly, and agreeing to make it part of a relationship. I understand that this could be considered a perversion in the literal sense, that it is different, not the norm. I don't think that I am "sick," I only involve with others who are of mindset similar to my own, keep it all above board, and I don't play with cheaters (I've turned down plenty). I also don't go to swing clubs or other anonymous sorts of things, though I don't judge those who do. It's not my scene. I like to know the people I play with, and understand their reputation, and know who their partners are. I personally know men who go to swingers clubs as a single man just to cheat on their partner at home, and that is not what I'm into.
I think that many swingers/poly people are more secure in their relationship because they have to be, because it can force a lot of communication that might not normally have happened, and because they are less likely to be codependent, but having been in this lifestyle for a long time, I've seen a lot of unhealthy situations, a lot of people pressured, a lot of people who have left their existing relationship to shack up with someone else... it does happen (hell I guess it's what's happening to me, though I don't blame the lifestyle). And it can complicate things. When it's done right, it is very free-ing. It's hard to describe. It can greatly enhance a relationship (again if done right). I've managed to become a part of a very close-knit group of friends, people who do all the normal things that people do with friends, like help them move and celebrate birthdays and watch each others' pets when we're out of town. We also happen to swap partners sometimes or play in groups, and yeah it's fun. And there is a lot of trust in our group, and we all play safe and get tested regularly. New people come and go (no pun intended), but the core group has known each other for years. And when someone new is brought in it is because someone in the group knows them and thought they would be a good fit. Reputation is very meaningful in this group, and being ethical is important for those who want to stay.
In my situation, the cheating happened with someone my WH and I were both seeing at the time (we're technically polyamorous, so while we do some swinging/orgy sort of activity, we also have full-on relationships outside our marriage), and it happened via them failing to use protection and agreeing to lie to me about it (about five years ago... when I first came on these boards). And although my husband and I were on the verge of full R, having been separated this whole time but basically dating each other for the past 3.5 years and getting ready to move back in with each other, I just found out that he's lying to me again, about meeting up with a couple for sex, a couple that I don't know about, and that he hasn't told me about or admitted about. So who knows what else he's been lying to me about. Our rules are that we tell each other. I did ask him directly about what he did that night and he said he stayed in to recover from a busy week at work. So here we go again, and this time I'm done.
All he had to do was tell me, and if it was going to be ongoing, for me to meet them just to make sure all was cool, and he couldn't manage that. Which is why I'm back on the boards because I'm about 90% sure we're headed for divorce now. There really, really needs to be trust in a marriage, regardless of whether or not arrangements are made for outside activities, and ArableSands hit the nail on the head - it's the lying, the betrayal that is crushing, the failing to consider me and the very few rules that we set up to be successful in this lifestyle.
@missymomma thanks for the clarification and recognizing probable bias. I know some people who would meet that description... I don't play with them. As for me, I probably have sex about four times per month (in fact being poly has helped a lot with this, since my partners can get their needs met away from me when I'm working long hours or otherwise too tired for sex, it really takes pressure off me to perform), it's all I have time for because I have a really demanding career, and when I go to parties with my group of people I sometimes go just to talk and catch up and don't play at all. But I am very much a hedonist, and I guess in the interest of shared values in relationships and friendships I tend to primarily be involved with other hedonists as friends and lovers. Anyway yes there are sex addicts who do this, though there are a lot of people are aren't sex addicts that do this. I would probably have more sex if I had more time for it.
@c6248x for me, it depends on the person. If someone I don't like is going after one of my partners, it does incite jealousy. If someone I know and trust goes after one of my partners, we've usually already discussed it and know that it's all good, and then I smile knowing my partner is having fun with someone else I care about because it's usually a mutual friend, or another established girlfriend that I've met and that we're aware of each other.
@DecadeCentrifuge... one of us, one of us 
Sometimes I feel like I love my partners even more when I see them so happy with someone else. And then they come to me and, eh, share the love. Personally I couldn't be with someone who expected total monogamy from me, I'm not wired that way, so I just don't mess with monogamous people. I like the option of screwing my friends but it isn't required for friendship, we just all understand the boundaries and clarify as needed. My husband and I would work great if he weren't lying to me. I also have a boyfriend who isn't a lying bastard, I always know what/who he is doing, and I always have an opportunity to express any concerns, which he listens to without accusing me of trying to control him. Unlike my husband. And I also have a girlfriend, who is married to a man, I don't play with him but we know and trust each other. She is now mostly monogamous, I am the only "outside" partner she has, I was grandfathered into their arrangement because I was involved with her long before he came around (I've been with her for six years, he's been with her for two), and because I respect their marriage as their priority. I don't mind taking a back seat, I'm just happy to see her so in love, and treated well by a great guy, and grateful that he trusts me and allows us to still play together, and if she felt that it would be best to transition to "just friends" I would abide by that because I love her and I am respectful. Mostly though I babysit their kids (we're discreet with the kids, they know me as a close family friend) and watch their dog when they're out of town and hang out with them, the relationship is hardly about sex, they have tons of it, she and I might only get down and dirty twice a year. And it's all good, because at the root of all this filthiness is genuine care for the people in our lives, and wanting what is best for them and their happiness.
I have a beautiful photograph... it is me, my husband, my girlfriend, my boyfriend, and a now-ex-boyfriend (who was also a boyfriend at the time), all standing with me when I graduated with my bachelor's degree, all of them having supported me emotionally through school, sometimes financially in a pinch, all of them hanging out with each other, all of them aware of the others' role in my life, all of us just smiling on a day that was important for me. No sex involved, just love. Real love.
I still don't understand how my husband managed to screw this up. Again. At least I have a strong support network because I have a feeling I will need it. But I guess I'm so used to spending so much time alone, it isn't like the first time, when we were living together. The first time, it was like my whole world was ripped out from under my feet. This time, I'm just disappointed with him and looking forward to moving on. I still love him, I suspect I always will, but I love myself too, and I deserve better.
