Hmmm. I don't know I agree with there being only two possible reasons. I went through a period of time (about two years) after the first time I had sex where I just had no desire for it. Prior to my first sexual encounter at 16 I was very curious and very into it and felt like it was something I needed to do or a right of passage. After, I just didn't see the big deal. I was still attracted to people and formed relationships and even romantic connections, but sex was just not on my to-do list. There were so many other things. Emotions, intellectual conversations, even physical contact not involving sexual contact. And there was nothing physically wrong with me nor did I have any psychological aversion to sex. I simply wasn't interested.
I've gone through phases like this on and off in my life, during which I would have considered myself as asexual. During some of these phases I DID engage in sex, not because I wanted or needed the sexual gratification, but because my partner at the time did and it gratified me to make my partner happy in whatever way I could, including sharing my body. But I didn't feel the usual physical pleasure associated with sex. I wasn't adverse to it. I simply could "take it or leave it", but would have left it if up to me.
Am I hitting the nail a bit closer to the head here StarGazer?