You have identified the expectation that females will share one bathroom and males the other as a customary arrangement, enforced by community expectations.
The custom of allocating bathing facilities on the basis of gender is common (although not absolute) in many societies, including Australia. Debra and Alisha’s expectation, without discussion, that this rule will apply in the shared house, appears to be a reliance on such customary behaviour. People may have the same expectations for other reasons, for example, as a result of religious beliefs or personal morality, but nothing in the given facts suggests that this is the case here.