Getting back to the OP's issues:
Originally Posted by 123
It took time to develop the emotional attachment I have with my boyfriend and he knows that at this point, the only real danger to our emotional connection is him pulling away and distancing himself from me [which he's done as a result of this issue].
he . . . worries any guy I'm with will want to steal me away. But he also understands that *I* also would need to want to leave him and he understands that I... am... polyamorous.
. . . he's been quite adamant about not wanting to develop relationships with anyone I've been with [quite opposite to what we've always agreed we both needed in a polyamorous relationship]. But he's always encouraged men and women alike to explore their sexuality and be open. His last two long-term relationships grew out of NSA relationships, a point he emphasizes often whenever people criticize the lifestyle choice -- he sees people being open about sexuality a sign of strength rather than weakness.
I really am baffled because on every level, he really is the guy I want and love. In every aspect he thinks/feels/acts in ways I respect and want in a partner... but somehow when it comes to ME, to his future wife, it switches up.
There definitely seems to be a conflict within himself that is coming out more fully, perhaps, as your relationship deepens toward a marriage. I still think this is a madonna/whore complex he's got going on, and if you do pursue therapy, that is where your focus should be, I would think. On such double standards, goose-and-gander type issues.
Originally Posted by 123
He really is... a million times better than anyone I ever thought I would find in my lifetime. I have very high standards . . . It's just this one issue alone that has made him switch up so drastically.
When I read the first sentence, I immediately thought you might have self-esteem issues to work on, but then you surprised me by saying you have high standards (and therefore that's why he' s surprised you by meeting most of your expectations). So it is interesting. I wonder if (bear with me, I'm just tossing out some ideas to consider) perhaps you set the highest standards purposely to make it difficult to find someone who will cherish you and treat you well. Yet you have still attracted someone who has met most of those standards and is now repulsed by the idea of you expressing your sexuality as you see fit. So, is there a belief you hold about yourself and what you deserve that a partner will eventually align themselves with to be with you? How much do you feel deserving of everything you want and hold dear?
We can't change the people in our lives but we can look at ourselves and sometimes we find that the ways in which we regard ourselves creates a certain dynamic that our loved ones get caught up in, like a whirlpool. Could be worth looking at.