Thank you all for your responses.
Some of them are exactly what I'm trying to get at: seeing what this phrase about it enhancing a marriage means to others. Scraping below the surface. I see some answers that make it more palatable and understandable than how I viewed the phrase.
Why don't I just ask him these questions? Well, first, this is what forums are for. There's not much point any of us being here talking to one another if we all just tell each other, "Go ask your spouse/boyfriend/girlfriend!"
We're here to talk about and explore ideas, get input, see how others view things in order to help ourselves in our own situations.
Second, I do ask him. He is unable to explain how he sees it. He gets agitated, defensive, and upset. He told me over and over, a week ago, repeatedly, that he has made it hard for me to be open because of his responses (this sort of came out of the blue, from him, it wasn't anything I brought up). He apologized, said he wants me to feel safe in talking to him, in being open with my feelings, so that he and I can be closer. So I asked him this, what I thought was a simple question, a simple matter of what does this phrase mean to you?
How do you
see it? It blew up in my face and he told me last night he guesses he really can't handle my questions.
I make no apologies for asking him questions. I have lived solidly in the world of traditional, monogamous marriages my entire life. I have no experience with open marriage or polyamory. I enjoy his company very much, I think we've been good for each other in many ways. I would like to continue seeing him. But it's a foreign world to me, I do not fully understand the mindset and I think I have every right to read, to educate myself, to try to understand, and to ask for explanations and clarification.
As one who has spent my life in education, I do not yell at my students for not understanding what they've never been exposed to. Sometimes, often, in fact, it takes going through a concept multiple times for students to really get and understand even 'straightforward' concepts like math equations, physics, sciences. Never mind the complexities of feelings, emotions, relationships, what it is to want to be with two people.
I have been respectful in all my questions, I am genuinely seeking understanding. I feel if he invited me into this world he knew was completely foreign to me, he has some obligation to help me through it if he wants me to stay here. Just as I would help a visitor to my city navigate the streets and sights. Especially someone I greatly cared about and hoped would come back and spend more and more time with me, and maybe even join me permanently.
I feel if he's this frustrated with what I thought was a rather simple, innocent question, he needs to think about why.
There are other issues raised here, such as feeling second, but right now, I have many other things I need to be doing, and given the discussions he and I are having, it may become a moot point, regardless. I'm looking at some of the reasons why this is bothering me now
despite the fact that I have no desire for any husband at all, not him, not anyone, despite the fact that I have no desire to move in with him or any of the rest. I have been quite happy with this situation for those reasons, and logically I ought to still be happy with it, as I still feel that way.
However, I'm seeing that some other things going on have raised that feeling to the surface, things I think it would be unwise to discuss on a public forum.
Again, thank you for the answers, and I'll be re-reading some of them and pondering them again.