And if Aden uses both?
Ah, good question, but the answer should be easy.
A Female would never use the HIS towel, in fact some females would not use either towel as they use the HIS & HER towels solely as decoration.
The Male would use the HIS towel or both if there was a situation requiring two towels, or he already used his towel and never bothered to throw it in the hamper to wash so uses the remaining towel.

So to recap:
NEITHER TOWEL USED= Female
ONLY HER TOWEL USED = Female
ONLY HIS TOWEL USED= Male
BOTH TOWELS USED= Male