As I recall, Bartholome's physical age is closer to puberty (11-12 versus Valentina's 6?), which may be a factor in why he's more interested in sex.

Given that LKH decided who was what age, though... ja, it's the gender roles, all right. That, and I think LKH has a secret thing for increasingly... underage... males that she doesn't have for females. See Kitto in the Merry Gentry series.