I’d say it if there is one room that is female only and one room that is male only, then gender_segregated=yes applies. And if there is one room that is gender-neutral then unisex=yes applies. It would be a contradictory combination applied to a single room, but applied to a POI node representing an unspecified number of rooms the logical conclusion is that there must be at least three separate rooms. So this should be clear enough to communicate the situation and distinguish it from these two other situations:
gender_segregated=yes+unisex=no- There is a male only space. There is a female only space. There is not a shared unisex space.gender_segregated=no+unisex=yes- There is neither a male only or a female only space. There is a shared unisex space.
This all makes sense for large public bathrooms/restrooms/toilets containing multiple individual stalls and a shared space for hand washing.
Where the meaning may get muddled is that sometimes public toilets are not shared spaces at all but rather a set of completely separate single occupancy, lockable rooms. Sometimes in this model all the rooms are labelled as gender neutral so it would seem that gender_segregated=no + unisex=yes would apply. But this situation is different than if there were one large gender neutral room shared by everyone. Each single occupancy room offers complete privacy for the person using it, so they will not be sharing space concurrently with another gender despite none of the rooms being designated as male or female. Some way to tag this difference would be good.