You sound very thoughtful and mature. If you're curious about trying to date other people, why not go for it?
I don't believe that people are either "mono" or "poly" in clear, exclusionary categories. I think some people are less likely to have emotional/romantic/sexual feelings for others when they are already in love with someone; and other people are more likely to have those feelings for more than one person at a time.
I don't have much advice for you except in regards to your feeling that when you have a lover--someone filling your "lover" category--you become blind to all other men. That's perfectly fine of course, but if you are curious about exploring poly relationships for yourself (and why not?), yes, you will definitely have to make a choice to go outside your comfort zone and date deliberately.
Your comments reminded me of how I felt when my first boyfriend broke up with me. I still had feelings for him months later. I was in love with him and felt that my "lover" slot was filled. I knew he didn't love me back, but I felt like my heart was already "given." I hardly took notice of other men. But I knew that I would have to push myself to date again to get over him.
So I did. I went outside my comfort zone. And it turns out, the world is full of interesting men! In fact, I ended up connecting with a non-monogamous man who understood that I was not over my ex. He became the lover and supportive friend I needed at the time. My feelings for my first boyfriend didn't disappear just because I'd found someone new to love. (Thankfully, I got over my first love eventually, after I matured, got more life experience, dated a LOT more, and realized how immature he'd been).
But, my point is, I had to push myself to date when it felt unnatural to do so, and it was absolutely the right choice.
I guess the main question for you would be whether falling in love with someone else would cause you to fall out of love with your primary boyfriend. Mono people seem to experience that, although I don't understand it at all.
I think there's a lot of societal conditioning that you can only love one person at a time, so when "mono" people fall in love with a second person, they have to convince themselves they don't love the first person anymore.
I understand your fears that your boyfriend may somehow want your mono love for him to remain mono. Why not talk to him about that? You may only be imagining that he feels that way.
Single, straight, female, solo, non-monogamous.