You can't try to apply everything you read about polyamory or open marriages to yourself. There are tons of different situations that people use the polyamory or open marriage label on and unfortunately some people write about themselves or their experiences as if they are the only definition.
It seems to be true that there are married couples that turn to the idea of opening as a way to fix something that is already lacking in their marriage. Also some couples may look at opening as a way to enhance their existing happy relationship. That doesn't speak for all of them.
In any case, even if they do fall in to the category of looking for enhancement, it doesn't mean that your relationship with your BF exists solely to enhance their marriage or that it should be at your expense.
Presumably you feel you are gaining something from the relationship or you really should consider why you are in it. What you read about other relationships shouldn't define your own. Does your BF or his wife make you feel like he is only with you for their marriage's benefit?
I can tell you that my relationship with my other partner never had anything to do with the intention to enhance my marriage so that statement does not fit me at all. In practice it has had both positive and negative effects on my marriage, none of which I went looking for. I have a relationship with him purely because I want to have a relationship with him so there is no balancing those ideas for me. If you asked me for help understanding couples who are open for that reason I wouldn't know what to tell you. So if he indeed doesn't see you that way, he may just be frustrated that you are asking him for help understanding something that he himself doesn't understand and doesn't apply to the two of you. The fact that he is open doesn't make him an expert on every kind of open relationship.
Instead of asking him about other people, ask him to explain more about your relationship. He shouldn't be fighting that. If you need more information about how he feels or what to expect from your relationship, ask for that. If he says "Don't you know how I feel?" Say "No I don't understand. It isn't clear to me because I'm used to the idea that relationships go a certain way so I need help seeing a new vision of what our relationship can be without being those things (fill in what your typical expectations would be)"
Once you have that vision, you have to decide if that can meet your wants and needs now. Then if your feelings change in the future you'll have to decide again.