Here's the tricky part. This guy doesn't know about me because he is not poly, so he would be hurt and jealous, (especially because I'm a woman).
At first, I felt uncomfortable about this, but felt more understanding when I knew a few more of the details. But, my original understanding was that they'd be breaking up during their visit and so far, the guy I'm dating hasn't had the courage to tell his boyfriend his true feelings. They've spent the last few weeks together and the boyfriend still doesn't know the truth!
This seems so dishonest to me and not poly at all! I feel uncomfortable about it.
The fact that it's making you uncomfortable means it's crossing a boundary or violating a value. That's pretty much how I define "cheating".
I've pretty much decided I'm steering clear of the whole messy situation and intend to be just friends when he comes back. But, I thought maybe one of you wise poly's might have some good poly advice for me.
My advice would be to talk to your partner. Use a lot of "I messages". From your perspective, I know how it feels to have discomfort and insecurity and I think it's important to honor that and communicate it to people you love.
I also know his side, the fear of being honest. Of hurting others, or being hurt yourself.
So... I'd say try not to "fix" anything. Relationships aren't static things that have a "should be this way", they're fluid, always changing. So you'll never "fix" the problem. But you can express how YOU feel and open the doors for him to do the same.
If you feel uncomfortable in a realtionship, there's something wrong. But "just being friends" doesn't address the problems. It does nothing to ease your own misgivings and it does nothing to make it easier for him to deal with his fears. Expression and honesty DO.
Sometimes, it takes someone else being honest and sharing to trigger the confidence other people need to do it themselves.
Also... as a mental preparation, be prepared for his response to be something you didn't count on. Perhaps you were a "fling" and you should be prepared to hear this. Perhaps he's battling issues with his own sexuality and he's not sure if he wants to be "bi". I'm just tossing out examples; not suggesting anything, but you should be warned that this might be the tip of the iceburg for him in terms of deep-seated issues and tough feelings.