Yeah, my ex-wife. She told me after she took all my money and half my stuff, but I had a pretty good idea when I found out she was sleeping with another woman while we were married.
Oh well… pardon my language, to hell with the bitch.
I’ve never had an S.O. go play for the other team, but I have a good friend who one day announced her crush on me.
That was a bit strange… considering a few days before she’d asked me for advice about her male S.O.
I still adore the girl - but did tell her I didn’t play for her team… unfortunately.
E.
Then there was a woman I know in college who, in the time that I knew her:
dated one man who later came out of the closet;
dated three men who later became Roman Catholic priests; and
dated one man who later married another woman.
We still rib the last guy. We would stop, but he is soooo cute when he gets defensive. 
Sua
I went out with a woman for a while, knowing that she was sexually ambivalent. I suppose that doesn’t count per se, but she did leave because she had a crush on another woman. She ended up marrying some other guy entirely.
One of my exes “changed over” not long after we broke up. Now, if they had been women she’d been sleeping around with while we were together, I could’ve chalked it up to some crisis-of-transition thing.
But they weren’t.
I broke up with one of my SO’s after my brother found… um… interesting artifacts hidden in his room (long story). I’d always known he was odd about gay people, but hadn’t figured out that he was in denial. A typical conversation with him would go something along the lines of:
Me: Can you pass the salt please?
Him: Sure, but I HATE F#@%IN’ FAGS!
Me: O…k…
He’s still W-A-A-A-Y in the closet, to the point that he got married last year. I actually bumped into someone who’d been at the wedding, and he (not knowing what I knew) commented that it was odd that the groom danced with the groomsmen all night instead of the bride But his father is a complete homophobe, so I doubt he’ll ever come out. He’s the kind that will try to deny it for as long as possible, and then top themselves because they have been raised to hate gay people. Believe me, his sexuality is the least of his problems. I’d have dumped him even if I didn’t suspect he was gay. He was a first class jerk!
My first serious boyfriend was gay. I knew the entire four years we were together (if you can call it that) but lacked the self-esteem to end the farce. Better someone than no one, I rationalized.
One day he came out to me over Japanese food. First he was bisexual. Six months later he was completely gay, if there is such a thing.
We don’t talk any more and I’m better off for it. I tend to think his sexuality has less significance than the fact that he was a raging jerk, but that’s a topic for an entirely other thread.
No, no SO ever switched sides on me, but I did it to an ex-SO. We’re all still friends.
Hint for men out there: if you’re going to have a threesome with your wife/girlfriend and another female, bear in mind that it may end with the two of them together and you, well, solo. Just another reason why the fantasy might be better off staying a fantasy, y’know?
Hey, so long as I can continue to operate the camcorder, fine by me.
Sua
Well the few priests and monks who I have had sex with would have some type of weekly epiphany. It wasn’t exactly switching sides so much as saying they would stop having sex with me. If any of you people out there have sex with a priest it seems to follow this pattern:
ME: Wow, another priest.
Priest: God, I will just do it this one last time and promise never to sin again.
Me: Let’s have fun.
Completely wild sex ensues with utter passion (they typically seriously believe they will never have sex again…so it seems).
Next week.
Me: Wow, the same priest again.
Priest: God, I will just do it this one last time and promise never to sin again.
Me: Let’s have fun.
Completely wild sex, etc.
I have also had a few unpleasant bi surprises. “Surprise, I am married to a woman.” Or in a case of a one night stand (thought it would be more but he gave me a fake phone number) finding out later that he works with me somewhere in my building. Even more so, he walked in with his wife and three kids (ouch) and ignored me. That ended any hope there. Closeted men who cheat on their wives just don’t do it for me. I would rather have someone willing to commit to me or at least be willing to share the fun one night stands get really boring and just turn people into jaded sluts like my friend Bill. (love ya, mean it, Bill. Mwuhahahaha) Still there is nothing wrong with being a hedonist. I guess I have to start that bad advice thread sometime. 
HUGS!
Sqrl
You know, my first thought on reading the OP and the subsequent posts was one of triumphant topsmanship. I had an experience that would beat all of your stories into the ground and stomp them into tiny narrative smithereens with the fury of a million suns (or half of an OIP meatball sandwich, one of the two).
But then I realized the timing was all wrong! Horrors! Whatever will I do now?
So I decided to tell the story anyway. It’s amusing, at least.
In junior high, I met this girl who liked all of the same things I liked…science fiction, fantasy, d&d…and, being something like 13 years old, we started dating. And dated for a year. Then broke up.
We went to different high schools (though a mutual friend went to my high school and became my best friend, and the reason my ex and I met up again) but ended up running in the same social group immediately after we all graduated high school. Then I went away to college and, while gone, admitted to myself (and others) that I was gay.
When I returned, I came out to my best friend (the mutual friend of my ex and I). Two weeks later, completely independently, my ex came out to this friend also! The friend’s reaction was basically, “What did you guys DO to each other?!”
Kelly (my ex) and I actually got along better as gay friends than we did as SO’s. She was the one who dragged me into the local gay bar for the first time when I turned 21. And she was (and still is, on the rare occasions that we’re both out at the bar at the same time) the one person I could introduce as “my ex-girlfriend, who turned out to be a lesbian.” She hasn’t hit me…yet…
jayjay