IMO, I see being intimate (whether it is kissing, intimate conversations or sex) as things that progress along a continuum and it is very difficult to stop at one specific point, as in "okay dear, I'm going out with Angela tonight. I will share intimate moments with her and hold hands but not kiss or feel emotionally attached to her beyond level 3." Yes, we can dissect physical intimacy and even emotional intimacy to some extent, but really it is all part of a fluid process.
If you like, check out my thread: http://www.polyamory.com/forum/showthread.php?t=1020
which illustrates my less than honorable way of discovering polyamory. The point is that no matter what I did or did not do with Maia, I still felt the same way about her. It didn't matter to Leto if I had slept with her or not, I had in her eyes betrayed our love because it was only supposed to be for us. I don't know if had I not kissed Maia, would Leto have still felt the same? I think she would have because rather than hearing me say "I love both of you." she was hearing "I don't love you that much because there is room in my heart for someone else." It is the difference between the mono and poly paradigms and language.
So your b/f isn't having sex w/ anyone else but he's flirting with someone. And he might want to have sex w/ her. Boy does this sound familiar. (me) I actually had a some really great dates/experiences while with my last g/f because she was having trouble with me being poly so I was trying to move slow (but also, not slow enough for her) and I was not having sex which was sort of new for me. When I was monogamous and single, I would typically jump into bed with a new partner within the first week, if not the first night! Having a partner already made me more cautious, slow and careful.. Not just because of her recaltriance, but also because becoming sexual and advancing to that level of intimacy was a bigger responsibility. Not just taking care of myself anymore, but thinking of my partner as well. Normally being a "dive into everything head first" sort of person this was a new and exciting approach for me.
The story didn't have a fairy tale ending however. My g/f ended up breaking up w/ me a couple weeks ago. No need for sympathy however, I think both of us are wonderful and resilient humans who will take the best from our experience and move forward. Maybe our paths will intermingle in the future? Her handle is Nyx if you want to check out the saga from her side as it is somewhat more akin to yours, I imagine.
As for your situation, I did not read but somewhat agree with the sentiments from the Ethical Slut. Of course, I'm poly and not very jealous, but I think that having your partner have sex with someone else isn't so bad. As long as your health is not at risk (protection, STI testing and disclosure, &c.) the biggest things are fear, IMO.
Fear that she will be a better lover or turn him on more. Fear that you will no longer turn him on. Fear that for that reason or for other reasons they will link up and leave you "out in the cold". That's monogamy. And that might very well be what happens. It's about trust. It's about knowing your b/f, and speculating what you can trust him to be like. Trusting yourself, knowing how you would feel if he DID leave.
My only issue with my g/f dating other men was all the men she found were monogamous and if they grew to like her most likely they would try to lead her away from her relationship with me. It wasn't really a big issue, because I trusted her to do what would make her most content. If that meant leaving me, then so be it. I might sound magnanimous there, but really I'm not. I'm just looking out for my own happiness and I couldn't be happy knowing she was miserable with me. That's part of why we're not together now; she gave many indications that remaining with me would make her miserable.
Sorry I'm so long winded. I could write a book.