Well, take a deep breath and try to disconnect emotionally.
You're caught on the same treadmill that so many people get caught on, so don't make more of it than necessary.
There's so many misconceptions about sexuality we are raised with in our culture that after awhile we begin to think of it as "normal" - whatever THAT is
But based on what you've written here so far, it's a PRIME example of one of the many ways that kind of thinking breaks down under analysis.
Sex has many aspects to it - all of them good as long as nobody is harmed and all consent. You've already seen that he is stimulated by the D/s aspect - you aren't. That's perfectly ok !
Your relationship (as he explained to you) is MORE than about sex ! As we hope all relationships are ! Sex is just one small component. With many sides to it.
No different than the fact that people we connect deeply with may have different appetites and tastes in other areas of their lives, food, movies, politics, music - you name it - so we can have different tastes in sexual stimulation.
Why should it be any different ? Because someone told us so ? Or we saw it in a movie ? Pleeeaaassseeeeeeeeeee ! Give logic a chance - right ?
The sexual pleasure we enjoy with different partners is DIFFERENT but NO LESS special ! Just as we have different likes in food we also have choices we share/enjoy TOGETHER !
There's room for both in our lives. Similarities AND differences.
But when we try to deny someone something they enjoy and that causes no harm, we're only driving a wedge into the connection we do have.
My suggestion ?
Understand it, celebrate it and let it go.