Conditioning. (Taken from
What Causes Infantilism?)
"The best-tested theory for the cause of fetishism is behavioral conditioning. It holds that items can become erotic if they become associated with sex. Experiments have exposed college students to pictures of neutral objects, such as shoes or coins, intermixed with heterosexually erotic pictures [ 8, 9]. These experiments were able to eroticise the neutral objects, but did not report developing of a compulsive fetish. This may be because the students were sexually somewhat mature or did not reinforce the connection over long periods of time. There are some anecdotal accounts of people becoming diaper lovers after erotic incidents that involved diapers. The reliability of these accounts is unknown."
Basically, as a baby, guys are able to achieve a sexual stimulus from a diaper easier than a girl. (I'm guessing at least, I have no idea how to compare, since I never had a vulva.) In our small, baby brains, we equate sexual pleasure with the feeling of a diaper.