Form what I’ve seen and heard, it’s because people like a slimmer physique in their characters. The reason some players choose female isn’t because it’d female, it’s because the game’s female player model is slimmer, and this is especially true for titans players because some players don’t want to wear extremely bulky armor and the armor is smaller on females. If destiny had the option to have skinny or muscular body types regardless of gender then male players wanting slim bodies would probably choose a skinny male.