I have always felt very similar to how you feel.
I'm a 22 year old male, and I've always kind of shunned intimacy. I've always been an introvert. I think that the only reason I developed as many friend as I had in highschool, was because I began smoking weed/drinking/partying. I never liked hugs, at all. Even as a young child, my parents tell me that I used to kind of shrug away when they would try to give me a hug or a kiss. I don't know why, but whenever someone tells me they love me (my mom for example) I kind of get mad, because I wish they wouldn't do that. I don't even like psychological intimacy. The first time I had sex I was 17, but that was only because she approached me outgoingly, and I thought why the hell not. If I tell my friends I'm not a virgin anymore they will think I'm somewhat normal. I was sober, and it wasn't all that great. I had a hard time keeping my interest (and she was a pretty girl, better than what most of my friends would get) I have never really been interested in sex I guess. I don't watch porn or anything, because it disgusts me.
Going through my late highschool years and earlier college years, I liked to party a lot with a group of male friends. They would all hook up with girls all of the time, and trust me they weren't the best looking. I'm not trying to be arrogant.. but I know I am above average looking because I've been praised for exceptional looks my whole life (which has always pissed me off, I honestly hate when someone complements me on how I look), but anyways, I would never hook up with any girls at parties even when they made advances towards me, and my friends all thought it was strange.
I was once convinced that I was asexual, but then I entered into a few year relationship with a girl and I realized that I am infact attracted to women. I recognize that even now.
I've analyzed the way I think in the past few years. I've realized that being praised for good looks my whole life has effected me in some way subconsciously, and I think that I probably just have really high standards for girlfriends/sexual partners because of that. I just don't understand how some of my friends could hook up with some of the girls that they have, and be proud of it/enjoyed it. I'll admit I have had a few one night stands while drunk or high on something, and I enjoyed it very much then, and thinking back now I think it was awesome. When I'm sober, there are very few people who I could see myself with, or could be attracted to. If I ever become attracted to a girl, it's at the initial sighting of her. I know right away if I think a girl is hot just by looking at her, but when I ever get closer to a girl, that attraction quickly fades away and I become disgusted about something, or grossed out. Porn is disgusting, and undignified, I just can't even watch it, AND I'm a young male..
I love the feeling, or the idea of sex, cause trust me I still have my instinctual urges.. but when push comes to shove and I know that I can have sexual relations with a girl that has given me all the signals, I will most likely not go for it. I have my own little bubble that I don't like people coming into, or getting to close. I can admire the way a girl looks from a distance, and think she's beautiful, but when the clothes come off there is something about that that turns me off.
Like I said I don't even really like becoming psychologically intimate with a girl and opening up to her, but I feel that the only way I could willingly have sex with her and enjoy every bit of it, is if I first become mentally and psychologically open with her, and kind of become boyfriend/girlfriend without anything physical.. and this has to be for a small extended period of time before I really know that I like her. From my experience, this is just how things work with me. I may not be sexually attracted to the girl, so to speak, when we begin getting to get close to each other mentally, but I will still be attracted to the way she looks and think she's hot, just not in a sexual way. I feel that over a month or two of a sexless relationship, and I really start to like her, that's when I really start burning inside and could really have sex with her all day and night.
I'm speaking from past experience in that paragraph above, and I think that's how it works for me I guess.
If I really don't like the girl as a person, I probably won't have sex with her because there's something that's just not attractive about her to me. Like I said before, I've gotten drunk before, or high on X before, and had a one night stand with a girl. It's happened, only a handful of times though. When I'm sober though, and I usually am, there is something about being that close to someone, seeing them up close, or feeling their touch or seeing them naked (even if they're good looking), smelling their breath, or having them touch me that makes me cringe, and turns me off.
Since I'm so introverted, I'm always in my own head thinking.. and the thought of having to ever share my thoughts and open up with someone else ever again is kind of frightening on a psychological level. I guess that somehow effects the way I am physically attracted to people too.
I'm no psychologist or anything.. but I'd guess that you probably need to become mentally attached to someone, and really like them, to become sexually attracted to them sober. That's how I am at least.
Sorry if that was a long ramble, but I took my prescribed adderall a little later than usual and finished up some work rather ahead of schedule, and the medicine was still in full effect. I just felt like I could relate a lot to what you typed, and I haven't heard anyone else express something that similar to what I have been feeling most of my life.