My husband was into the idea of other men sleeping with me, and I felt dubious about it but had been a very sexually adventurous sort back in the day, so thought I'd be game to try.
Went on a personals website and reactivated my profile (with his permission), had a couple of abortive attempts at threesomes that I didn't feel very comfortable with, plus was feeling more and more deep incompatibility with my husband. He had a recurring psychotic disorder triggered by sleep deprivation, with no interest in treating it, and I wanted children one day. But he also didn't work, and I felt responsible for his well-being, so I stayed and tried to make it work (and make sure he got enough sleep, always).
Anyhow, one day I met someone wonderful on that personals website. He was in an open relationship that was stagnant and one-sided (I was skeptical of this until I later heard ALL of his friends and family reiterate what a very lazy person this woman was, and exploitative, and how very unlike anyone else he'd ever dated in that way).
It turned out, against all odds, we'd known each other when we were barely more than children, and we were instantly inseparable. I didn't tell my then-husband that I was interested in the new man, even though he'd have probably thought it was hot. Something about the depth of my affection made me hide it. When I revealed what was happening to my husband, it was to tell him that I was planning to leave.
Tears ensued. I felt awful, of course, how could I not? But with his psychosis there was no way he'd have ever been the father of my children, I knew that. Somehow it was much easier to tell him I'd found someone new than to tell him that his brain was too broken, and that our future had dissolved the day we realized the connection between sleep deprivation and his episodes of psychosis.
He's remarried, presumably happily, to a woman 8 years my (and his) junior. I am happily married to my affair partner, who is more my match than anyone I could imagine, and with whom I have two gorgeous kids and hoping for more.
Because of how we got together, we've always had a fairly open-book policy about twinges of jealousy or things like that. If we think someone has a friend who's angling to get too close, we talk about it for a bit and figure out how to resolve it.
We also wouldn't hide accounts from one another, or passwords, not that either of us are particularly nosy but we've availed ourselves of it once or twice. We figure that's part of the price we pay for taking the risky path to happiness. We both knew it was risky when we embarked on it, we just figured it was a gamble that would pay off, and we seem to have been right. Our compatibility is off the charts in a way we haven't experienced in prior relationships, and we're much more secure and connected in our relationship with each year that passes. Sometimes we marvel at how far we've come.
Each of us had spent, prior to getting together, something like 7+ years being openly non-monogamous in various configurations: I'd been in a long-term poly quad and triad, he'd been in a long-term triad and had been a secondary partner to a relationship between two predominantly-homosexual bisexual women, among other things.
Somehow after we got together neither of us really wanted that any more. We slid into monogamy like it was an old, comfortable pair of shoes that had been sitting in the closet for a year unworn because of how unfashionable they'd become. I can't help but think that the secret's in the sex: both of our drives were (and are) very high and we're good at it, and we don't get bored with getting better at making each other feel good. We started out very compatible in bed, and we've been making considerable effort to learn and engage with the other's exact sexual desires for so many years, that an affair is almost unthinkable: imagine how long it would take someone else to get that good! Imagine risking your relationship on someone who might not even be any good in bed!