If any psyche majors have any ideas on what causes this kinda thing, please post reasons for male genital mutilation. I'm slightly curious
Not a psych major, but my partner was, and just about anyone I've ever seen seriously has been a psych major... and I've read a fair bit on various forms of what were once simply categorized as perversions. It seems the standard line, hugely simplified is that this sort of desire arises most often from a learned form of self hatred that is often at least a partial by-product of childhood abuse, which may be sexual, physical, mental or all of the above.
Personally I would also not automatically accept at face value his claims not to be transgendered, or to having other desires outside the socially-enforced norm, though I would definitely respect his own views on that, and his right to self-definition, and leave open the possibility that his desire is more related to a general anxiety about sex and/or male genitalia, rather than a playing out of conflicts over gender identity or affectional orientation.
Without knowing the guy, I would guess there's a very good chance, whatever his orientation and gender identity might be, that he grew up in a subculture and family more repressive than was common during his childhood. This might be a culture and environment that heavily frowned on gays, drag queens, transsexuals and anything else that tends to be associated (inaccurately in some instances) with male homosexuality and the stigma that carries in some American subcultures.
It is possible that he could be genuinely straight, and just hugely conflicted about sex, or about the messages he has internalized relating to male genitalia, stereotypes about expected male behavior or the like.
It would probably take months (or years) of close work with a very good therapist for him to be able to give you or anyone else a really clear picture of his motives. Chances are he's not entirely clear about them even to himself.
A desire for genital mutilation, genital amputation or castration, however, was reported at least as far back as Freud's seminal Schreber case, and has a long history before that, stretching back at least as far as the earliest development of cities and civilizations. Hard to say whether or not it predates the invention of civilization and of the sexual division of labor, which often seem to begin with gender-based divisions, and multiply from there.
As with the literature on lesbian identities, there are often degrees of confusion in motives, since in some cultures some forms of male genital mutilation were a prerequisite for certain highly desirable jobs (court eunuchs, for instance, in many Chinese dynasties and under the Byzantine empire, the castrati of Renaissance Italy or the (less high-status) hijra of India, to offer a far from complete list). One could legitimately wonder, at least in some cases, whether the desire was something inborn to the individual that the culture simply made a place for, or was coerced by parents and caregivers, or was a path chosen on more practical grounds.