Is there a rule that there be a women's and men's restroom?

Answer 1

No unless if it is a public place that has a fair number of people. Some restaurants only have one toilet/restroom so females and males will just have to share, your house only has one or two restrooms and you ( hopefully) dont have a sign saying female or male restroom so no. But you can call someone to have a look and check if your not sure if your place is considered as having a constant number of people.