I think there are a few people here who have mentioned having dabbled, at some point, in 'alternate lifestyles'. I'm one of them, and speak only for myself. I suspect you may find a bit of what I say surprising.
After my ex-wife's first affair, we dabbled in the 'swinging' lifestyle. We were fairly mild, in that we only saw one other couple. Some folks get way out there crazy, even more weird than you might already think, but we weren't interested in that sort of thing. It was something we thought to dabble with, considering things were already broken. I don't think we would have ever gone there had she not already stepped out. The affair really took away the 'sacred' feel of things for me.
Our agreement was very simple. It would always be WE, never any independent stuff, and if anyone was uncomfortable, it would end immediately.
We became fast friends with the other couple. The relationship was very different than I expected. To this day, my best friend is the other guy from that couple, but we are both divorced from our wives at the time, and have moved on to other relationships, and want no part of those things anymore. My tale, you can find in my post history. As for him, his wife became obsessed with pressing the boundaries, to the point that it became more important to her than her marriage and her child. She was deceiving him and meeting strangers, all the while lying to him. He ended up giving her an ultimatum: we either stop this, or I am ending this and finding a wife who will be monogamous. She chose the latter.
What you may find surprising is that both he and I felt, for various reasons, dragged into the situation by the behavior of our wives, rather than being the instigators. For me, I know it was at least partly with the idea of evening the score for the affair.
To answer your question about infidelity, yes, it still is a valid concept, at least if there have been any boundaries set by the people involved. It still hurts just as bad. Having been betrayed in both situations, I might even go out on a limb and say it hurts WORSE. It did for me, and it did for my friend when it happened to him later. He was absolutely devastated. Why? Because, even with alternatives, our wives STILL chose to betray us, and it could not be attributed to lust. It seemed that the betrayal, not the sex, was what they actually craved.
In the end, I think most BS's will agree that the WS's sexual behavior was not the source of the greatest pain: it was the lies, the deception, the certain knowledge that the person they trusted so much stuck a knife in their back, shared their secrets with others, made them less than an equal partner. It was the gut churning fear of trusting that person again, knowing what they were capable of.
I think, as some others have said, that monogamy is NOT the 'natural state' of people. We wouldn't have marriage at all if it was. We would not need to swear oaths of faith if we did not recognize the natural tendency is not to keep faith. But 'natural' and 'best' are two very different things. Our natural tendencies lead us to do much evil: violence, treachery, etc. We all have a bestial nature. Most of our morals are all about controlling and not giving in to those impulses.
One of the worst things that causes affairs, in my mind, is not recognizing that we do have such impulses. As Dr. Harley notes, everyone is capable of having an affair if they put themselves in a bad spot. There are so many who say, after the fact, "I never thought I was that kind of person." But they are. We all are. We have to guard ourselves, all the while living in a society that poo-poo's such precautions, and calls a partner who wants them in place 'controlling'.
Bah, I'm blathering, as has been pointed out to me before.
