I can't think sexually of people I'm close to?

I have a somewhat strange case. I'm a 17 year old male and I've been in what would be best called dysthymia for years, and I have depressive episodes over the most minor of things. I have never dated and I have had remarkably few crushes in my life. The thing is, I can't feel sexual attraction to anyone that I'm close to. I have mostly female friends, and although I can say that some of them are attractive, I am not sexually attracted to any of them, and I could never date them. I am very close with some of my female friends, in fact, all of my best friends are female, and with one exception, females are the only people I can confide in. That said, I confide in relatively few people. Of the relatively few crushes that I have had, I've only ever told anyone about two of them, and only after much prodding. I always hold in immense fear what would happen if said person found out I had a crush on them. This is strange, as I convey a rather quirky, unconventional persona and rarely care at all what most people think of me. I have toyed with the idea that I am asexual, but this isn't the case. I masturbate occasionally and sometimes (rarely) I watch internet porn. I honestly feel mildly attracted to both men and women, but I would never act on my attraction to men; I am primarily attracted to women and would identify myself as straight. (I sort of identify with the Kinsey scale of sexuality, on which I would be more on the straight side but still mixed.) But, I still cannot imagine entering into a romantic or sexual relationship with anyone that I am close to. I alternate between hating meeting new people and being a social butterfly with new people, the former being more common. I have always wanted to get married and have kids, but I've always pictured the marriage as more a very close friendship, and I have only rarely thought of the sexual elements. In a few cases, even dating back to 2nd-3rd grade, I have outright rejected romantic attempts from close female friends, because I was incapable of thinking of them that way. In short, WHAT THE F--K IS WRONG WITH ME?