So it turns out that she was originally drawn to him because she was unhappy in our relationship. At the time she wasn't really aware of what was motivating her.
Also, as for the sex but no love thing, I was actually very cool with the idea of her actually "dating" him, with healthy loving feelings and real bonding, so long as her and I would still be "primary" in the sense that we'd be long term, having kids, etc.
And the sex with him was bad not because he wanted more emotionally - it was just bad because it was bad. She never told him the sex was bad, and he never said that the sex would be better if she would be with him exclusively.
She's been taking time off both of us, and dealing with other personal issues, and has been struggling to figure out what she wants. After spending time with me a couple day ago, she seems to have flipped towards me.
She's also seen a couple of women in the mean time (having never done that before, but always wanting to try). Both of the women are bisexual and have boyfriends. She found the experiences healthy, erotic, and uncomplicated. She says she feels that she only wants to see other women while we're together.
I don't think she's ready to explore other men right now. I think she needs to develop a more acute awareness of what her needs and wants are before that step could ever be taken.