Nice fanfic.
Real reason - Loby is impotent
I know you are one of this forum's jesters, but in case someone truly cares about this topic:
—Hancock states Momonga wasn't turned into stone because he could force the lust out of his mind through pain > therefore you can avoid her fruit if you can free your mind from wicked thoughts.
—Momonga mutters that his petrified subordinates were fools > therefore someone who isn't a fool, ergo weak-minded, won't be petrified so easily.
—Luffy wasn't affected by the Mero Mero because he wasn't entranced by her (SBS 54) and has a pure heart > therefore if you have no thoughts of lust to begin with you won't be turned into stone.
I mean, it's obvious you won't be affected if your mind is strong enough to control the lust. Quite basic logic.
On a different note, you can be impotent and still feel lust. Or you don't know how your weeny works, buddy?