As everyone has figured out by now, the real issue is is that she isn't being completely honest. The question is, why? You've both agreed that it's okay to be involved with others. It could be a lot of things, but my best guess is that she feels that what she is doing is not okay. What reasons could she have for this? When she does tell you, how do you react? Do you give any indication that you're not happy with the arrangement? As you said, the main issue is her being dishonest. Maybe she's taking your irritation at her for not being completely honest as being upset that she's with other men in general, and she feels that she's rubbing it in your face and decides not to tell you? I get the impression that communication isn't great between you and her. Work on that. With monogamy, communication is the difference between a good relationship and a bad relationship. With polyamory, communication is the difference between good relationships and complete fucking disaster.
As for the "it's easier for women" situation: Not necessarily. From my own experiences, it seems like women are less likely to be accepting of this kind of thing if they're not used to it, but also less likely to cause problems later on. I'm currently involved with one woman who is also polyamorous, and every guy she dates says they're cool with the poly situation, and then immediately try their damnedest it get me out of the picture. The other women I'm "involved" with are hesitant to acknowledge anything more that friendship with me because they're still getting used to the idea of polyamory and trying to determine if it's something they want. Still, none of them has mode the slightest attempt to interfere with my other relationships. So yeah, women are less likely to be accepting of polyamory, but if they decide they are cool with it, they're more likely to actually be cool with it.
Another example from my own life. When I first met one of my "friends," I did not think of her as someone I'd be romantically involved with. We were discussing relationships and how we're both terrible at them, and I mentioned I was polyamorous. She did not react well. She assumed I was a "typical male," concerned with sticking my penis into as many willing females as I could find without concern for their feelings, and whatever else a "typical male" is. However, rather than keep that to herself and stop talking to me, she asked me questions. We discussed it for an hour or so, and at the end of the conversation, she said: "Hmm, that might actually work for me. I'm no where near being able to do it, and I'd have to transition very slowly, but I think there's a possibility that that's what I want." Surprised the hell out of me. So, anyway, it's probably not as hopeless as it seems.