Seems that his wife of many years had just started exploring the notion of having a partner outside their marriage (with his knowledge and blessing), and her new lover had managed to do some things with her sexually that totally blew her out of the water and circumvented some barriers that had always been present in their marriage.
The person posting about this was very distressed and upset about it, to the point where he was considering asking his wife to cut things off with her new lover.
And I think that's really interesting. Because upon reading his post, my first thought was "Dude! ROCK! You just hit the poly lotto jackpot! This is exactly one of the best things that can happen in a poly relationship!"
See, here's the deal. If one of my partners has an amazing, mind-blowing, life-altering sexual experience with some other guy, particularly an mind-blowing, life-altering sexual experience that brings down some barrier or opens some new door (and yes, this has happened), I'm all like, awesome!
For me, one of the many (many!) benefits to polyamory is that it improves my sex life.
And I don't mean "improves my sex life" in the sense of "lets me sleep with a bunch of women," but rather "improves my sex life" in the sense of "offers new avenues of exploration and new ways to find intimacy with my lover."
See, no matter how many things you can think of to do sexually (and as a seasoned, veteran pervert, I can think of quite a few), and no matter what you explore with your lover, the fact is that there will always be things that didn't occur to you and there will always be things that you don't explore. That's the way it goes; as human beings, we can not possibly ever do it all--not even if we live to be a thousand years old.
Because of that, there will always be doorways that we don't see.
This is especially true in relationships where some kind of barrier exists between the people involved. These barriers might take many forms--perhaps issues with relaxing and letting go during sex, perhaps problems with sexual communication or expectations, whatever.
When some new lover arrives on the scene, and explores something new or finds some way to bypass those boundaries, everyone wins. If one of my partners has a lover who gives her this awesome experience, then she has something she can take back into her relationship with me--"Hey Franklin! Check this out! If you do this, and then this and then this over here, then my body does this amazing thing! Isn't that cool?"
But more importantly, if someone is able to communicate with one of my lovers on a level that I never have, or finds a way around some kind of barrier that's always existed between us, then that person has just offered a gift of incalculable value. He's just created a roadmap to greater intimacy with my partner, by showing both of us that this barrier can be circumvented, and showing us how.
Now, it's true that some issues between people might be specific and unique to them. Even so, sometimes all it takes to begin to work on them anew is the feeling that it is possible to have a sexual relationship in which this whatever-it-is problem doesn't exist; funny thing about people is that when you show them something's possible, often that's all it takes for them to find a way to do it.
Plus, y'know, I really dig my partners, and I like when they're happy.
So to me, when a lover has some amazing, mind-blowing experience with someone else, that's a cause for celebration, rather than fear and angst. That seems to be a minority opinion, though--and that's a damn shame. Seems to me life is just a whole lot better when you're not all like "I have to be the best lover my partner has ever had or OMFG FAIL and I'm now worthless as a human being and she doesn't need me any more and brain weasel brain weasel brain weasel."