What Causes Lack Of Libido?

Dear Doctor,

I am a 29 year old male. I am an introvert person. I wonder until now, for my whole life, I never get stimulated when I see part(s) of woman body, even she is naked and I saw her many time for years. I know what sex is. But, every time I see it, I wonder and said to my self, "how disgusting it is ? how they can do that ? yuckk .. and so on". I wonder what is happening to me. I obviously do not want to be a singular grandpa in the future.

Hope for your explanation.

Thanks.