I hear a lot from women on this issue, which makes me think it has something to do with pregnancy and, self-esteem. When youre embarassed about your looks, you dont really want to be naked.
medication has been mentioned a lot, but that depends on the person.

Im 20/M and a Virgin. I dont take medication. Never abused as a child. Im ok with my body, sometimes doubt myself, but Im not overwhelmingly self-conscious about myself that I dont want to be naked. Yet, Sex just isnt a big deal to me. saddly, not enough of a deal to help me start a relationship... Im not against it, so I dont think Im Asexual. Im just not really big on it, its like holding hands, or kissing. It would be nice, but its nothing that Im going to make an ordeal about.