In my cognition, even if they are all zako fetish, there are many different starting points.
For many zako fans, zako is a kind of attribute, just like the commonly mentioned "loli", "tomboy" and so on among anime fans. They notice and are fascinated by the "common" and "pitiful" characteristics of zako. It's just that these characteristics are often better reflected under the premise of fight scene.
To some people, they reflected in the zako character (okay, I admit I'm one of them). We might have a bit of a masochistic streak, enjoying zako's mediocre and seemingly insignificant status, being treated violently and indifferently, yet still being chosen and loved in the end. We can feel a sense of empathy from such a zako.
Of course, others may simply enjoy violence, sex, or for various other reasons.
I can't speak for everyone else, but at least for me, I don't bring zako fetish into my real life. Don't view anyone, including myself, through that lens. It's just a preference for a certain type of artistic work. I'm not seeking, nor do I want, any connection between my partner and zako fetish.What I got in the Zako Fetish community is already enough. I have other interests and loves outside of it, and still want get along with her in a "vanilla way" . But I have to admit, I haven't had the courage to confess my hobby to her yet, because afraid of being misunderstood as someone who likes violence and hates women. I think that's a common misconception about zako fetish.
Just relax a bit, zako fetish is really... not that strange (I've seen much stranger things). Talk more with your BF. Maybe he's also the type of person who doesn't want to bring it into real life, or maybe he just said it out of shame for hiding it.
I'm not a native English speaker, so my words might a bit odd. Hope you can understand the general meaning. And also hope this can help you.