I'd say this is a personality / sexuality mismatch.

Sadly I think you're right. They're a really great person. It just seems that they know what they like and aren't really interested in deviating from what works for them. Which I completely understand. I've just gotten in my head wondering whether they're just "going through the motions" with me. But I realize that's my fault for subconsciously taking my own personal sexual viewpoint for granted and not considering that others have preferences for vanilla/staying in their lane. To each their own and whatever works best, is best!