The explanation why the bathroom door is locked and Liam cannot enter the room is relatively simple. Even though Sophia greeted Harris in relatively suggestive clothing, she can still explain that she wanted to prevent Harris from "straying" into the bathroom in search of a restroom.No one will disturb them.
If Liam tries to enter the bathroom, Sophia cannot stop him. And if Liam sees Sophia and Dylan bathing together, he will demand an immediate divorce.
...
So my wife would do the same if I had colleagues at home and she was taking a bath. And I would not find anything strange about it.
Sure the Parkers' house has multiple bathrooms, but I think my wife would still lock herself in.