To understand this question, consider the following theory: A boy who has worn diapers for all of his childhood is apt to develop sexual feelings
towards his diapers and other boys who wear diapers. As he gets older he may realize how odd or unusual this is when most other boys are sexually aroused by girls and only girls. To either explore these heterosexual feelings,
or to attempt to convince himself that he is "normal", the boy may focus on naked girls (thong-clad girls, including sisters) and other sexual thoughts that are typical for a preteen boy,
perhaps to the extent of attempting to supplant his long-established diaper-sexual feelings with "normal" heterosexual feelings. When the boy finds that girls are sexually inaccessible to him and he undergoes frustration in the face of
he may revert to what he feels is a tried-and-true outlet for his sexual feelings, his diapers and the boys who wear them. He admits that his sexuality is too deeply rooted in his diapers to ever change, but he may be
reluctant to talk to anyone about it for any of several possible reasons (e.g. suspicions of homosexuality, loss of friends, etc.). As a result, his sexuality is irreversibly skewed
and distorted by the ways his diapers influenced and affected the development of his sexual feelings.
Additionally, the boy may have confused feelings about how he perceives boys' butts and girls' butts. Boys' butts are smelly, messy and stinky
because of poop and farts, and while he knows that girls also poop and fart, he's less likely to have seen her do either, and that he has noticed girls are generally much more reserved about such matters.
He notes that girls also present their butts as a display of sex appeal once they are older. These are feelings the boy recognizes as he approaches sexual maturity. You
wearing a thong brings these concerns to the forefront of his sexual thoughts and he finds himself to be conflicted between his initial
impressions of the butt that, from likely only being around other boys bare butts at that age, he has held since his early childhood, and his heterosexual impressions of girls' butts where poop, farts
and the associated smells are usually not considered within the sexual context in which a girl's butt is presented.