I think that perspective is fine, and if Rien at some point had found a woman who could return her love or whom she could make love to, I wouldn't have had a problem with it. But taken together with the other problems in Rien's portrayal, it seemed to me that the idea that Mallory must retain a penis was solely so that Rien would not be making love to someone with female genitalia alone.
I thought Mallory (keep wanting to write Mal, then my brain jumps to Firefly, then my brain explodes a little) retained a penis because Mal's a hermaphrodite, and "I don't love you . . . you shouldn't change yourself for me" ? Unless you mean constructing the character Mallory as a hermaphrodite in the first place - huh. I thought it was just appropriately "not what it seems/contains everything," like his hold.