Thanks - a long response!
Thanks for your responses. I wrote out a long detailed history, but then I realized it probably wasn't all necessary and actually was more about just me needing to be cathartic. Then, I actually ended up writing out a super long response anyway!

In general, the problem I see with our communication is that I think he is holding back information until his conscience gets the best of him and then he shares with me, which to me says that he's not totally okay with his actions, but wants me to be. Wouldn't he just tell me if he actually felt he was doing the right thing? I don't know if he's holding back because he's afraid of hurting my feelings or because he doesn't want to take my feelings into account at all, or is it because his other girlfriend is manipulating him?
Here are some specifics -
we've been dating 3 months. Right away we talked about how we both wanted kids. We're both around 40, and for me it's kind of do-or-don't crunch time - I think a major reason I've been willing to be less rigid about who I date. But I am NOT just looking for a baby-daddy. I will be really sad if it doesn't happen but I am not willing to have a kid with just anyone.
I didn't know he was poly until after we'd been dating for at least a month. His telling me about his alternative lifestyle seemed to come in waves - starting with his attraction to older women (his ex was older), then to bdsm, then group sex and swinging with certain couples, then another girlfriend he had met recently before me at Saguaro Man, a local burn. During the second month of our relationship, these details sort of emerged one by one over a couple of weeks. Since I really liked him by that point, and I'm fairly open and adventurous, and maybe because I didn't know to what extent these relationships meant to him (since he hadn't told me about them earlier on maybe?), I went along with it, in spite of having put on my online dating profile
where we met that polyamory was not something I was interested in. I asked him about this and I can't remember exactly what he said, but it was along the lines of wanting me to give him a chance and not wanting to scare me off right away. He thought that I wouldn't give him a chance if he put it on his profile - that to me felt very much like a lie of omission. For something that is more and more obviously very important to him, I think he should've been more up front about it.
Another example of what I perceive as a communication problem - over the course of our relationship, as various things have happened, I think I have made it clear that I need time to process new things. I'm not overly assertive about my reactions in part because I don't always recognize them right away and need time to put words to the feelings. He knows this as it has happened several times (it took me a day and a half to tell him how I felt about him having sex with his girlfriend in the same bed as us).
This week we were both traveling - me for work and him for fun (with his girlfriend, to Burning Man). Last weekend, I was already feeling vulnerable and afraid about him going away for two weeks with someone else, and some things happened with his plans for preparation and we had a sort of argument about his other girlfriend taking up a lot of his time right before they were going to be away together. I felt neglected and told him so, and we ended up spending the bulk of the weekend together (I was grateful). Then, a day before I was supposed to leave, and when I was in the middle of very stressful work, he asked if he could come over. Silly me, I was excited to see him because I thought that he was feeling anxious about leaving me and not getting to see me for 2 weeks (like I was feeling about him). But he actually wanted to tell me that he was planning to have sex without condoms with his girlfriend (we have been fluid bonded for about 6 weeks and he made it clear that he was not fluid bonded with anyone else. We had tests etc.). In fact, his other girlfriend had given him an ultimatum that he would not be invited to sleep with her if he was planning to use condoms. He then wanted to give me all the reasons why it was the right decision - he trusts her, she doesn't have penetrative sex with her husband, she gets chaffing from condoms, etc. He is very focused on the "safety" aspect of it, but for me it signals something more intimate, and I had a problem with his willingness to let her decide for him (he made it seem that way anyway. Like it was out of his hands. But honestly I wonder if that was a way of deferring responsibility for something that he actually really wanted to do. I don't know if I trust him to tell me the real reason.)
He was obviously feeling anxious about telling me - and he said that he was even thinking that he wouldn't go. But this is Burning Man, and he had been very excited about going for a long time. His other girlfriend is the manager of a big camp and he is staying in her posh camper, out of the sand and wind, and he likes being her stud and will certainly shine as a stud while there. He kept telling me how grateful he was that I was supportive about him going, though that felt very manipulative - like telling someone you're really happy they're so happy for you when actually they're crying. I felt like he put me in a bad position to insist that he DID go, and that's what I said, and I meant it.
My immediate initial response was that I would not have unprotected sex with him if he had unprotected sex with someone else. But then he said that he wished I would just listen to him and not say anything. He just wanted to talk at me. (But in the past, when something has bothered me, he has gotten upset that I didn't immediately tell him what I was feeling.)
I can't stress enough how stressful the two days before my trip were. I had shirked all of my work duties in order to maximize our time together, and then was scrambling to finish everything, but instead we spent hours discussing him having unprotected sex with DR. He told me he had been thinking about it for a long time, and that his therapist had told him not to tell me, but that he was feeling guilty because "that's not what polyamory is all about."
So, although I do think we have different expectations for our relationship, and I am still working very much from a monogamous view, my main problem is that I don't know how to begin to trust him when he doesn't tell me these very important things about himself. And I don't know what he means when he says I can trust him (trust him to do what?). It also feels like this other woman is able to give him ultimatums and dictate when we are together, which he happily acquiesces to, but he insists that WE not have rules, he doesn't like hierarchies, he doesn't want ME to have any sort of veto power. (But then he still says he see me as being more important or having a deeper relationship, because he sees us as having a future and wants us to have kids.) That seems paradoxical to me. I understand Burning Man is an exceptional experience, and partly I feel neglected and left out, and taken for granted. In other situations, I have felt like he expected me to wait for him at home, not ask about his plans, not make any demands, while he has to show the utmost respect for the other people he is having sex with. (Specifically, he texts other people while we're hanging out, but when I asked him if he would check in with me cause it felt like he went into a black hole on the nights he slept with others, he said he thought that it was rude to text when he's with someone.)
He says that he wants us to be able to talk about things, but then it FEELS more like him telling me what he's going to do and expecting me to have a certain kind of reaction or no reaction at all. He wants me to feel compersion, but I don't believe that I can force myself to feel that. I believe that it takes time and that trust has to be built. And he apparently doesn't trust me either, or he would have told me earlier.
Writing that made me realize that maybe this is just an exercise in our building trust in each other, and it could be a really positive thing. But I'm afraid that he will get the message that I am easily placated and that he can really just do anything - that his actions don't have consequences for me, whether real like STIs, or abstract like hurting my feelings. I'm afraid of being taken for granted, and I'm afraid of wasting time trying to be flexible only to end up broken hearted anyway. (The fact that I'm almost 40 and want to have kids should be an obvious factor in my fear - I spent a lot of my past afraid to settle, and now I'm tired and afraid of being alone. I have been trying to work on that for the past couple of years but I haven't gotten to the place where I feel comfortable making decisions when relationships get hard.)
And while I really care about him, and we have a lot of shared values and shared life goals, I am conflicted about how much time to devote to developing our relationship when what I perceive as our main issue doesn't seem to be getting easier. Does it get easier? Is it too early in our relationship to know? What if I'm just mono? It's so early still, but it feels long enough for me to say I'd rather refrain from unprotected sex with someone else in order to protect something I see as having potential to fulfill one of my life goals; and it hurts me that he doesn't feel that way.