Artist needs to underline somehow that thouse creatures were males somewhen, reduced now to the pitiful and/or enviable state as they are. Given that this is - what, third? fourth? - not a first part of the series, loosely connected etudes as thouse may be, the reminder must be set in place, the degeneration and submissiveness molding being the focal fetish, and all. Beside that, I presume thouse small penises are one of the primary fetish aspects of the target audience, rest of literally everything be damned.
Teal deer: to Hell with biology, this is PORN! Long live Willful Suspension Of Disbelief!