As an Ace, I often fantasize about intimacy,getting close sex even. To complicate matters I am a sub. That's not really relevant except to let you know my fantasies can be extreme.
However I have no interest in sex as a function,as in the act itself. In fact these thoughts exists quite happily in my head and are often not something that interests me when the actual opportunity arises.

I put this down to my stimulation being about 90% mental and only 10% physical.
I understand you have a different issue
I am wondering if you have considered how much is the mental verses the physical.

It sounds very much like the natural apprehension of allowing another person to be there at your most exposed and in some sense vulnerable,coupled with some underlyingshame surrounding sex/sexuality,etc. Probably subconsciously drummed into you as a kid.
Can you pin point, what it is you feel ashamed or/dirty about?

I do hope someone can shed some light, apologies for wittering on.