Pathological self-denial in the sexual arena; refusing sex or sexual overtones even when they are clearly beneficial.
As with conventional anorexia, sexual anorexia is a complex nexus involving cultural expectations, parental pressure, peer pressure and media depictions. Once established it creates (possibly psychosomatic) physical, medical and particularly behavioural reinforcement measures.
As with many conditions (including anorexia) the best chances for recovery are offered by treatment in combination with the love, understanding and emotional support of those in one's life.
I've had some negative feeback about this. The above is not meant to suggest that sex and sexual overtones are automatically bad. I'm not saying that there aren't good reasons for some people not to have sex, merely that pathological self-denial is bad. Look up Pathological.