This really applies to everyone, not just Steve and Karen. Everybody, when you come to my apartment, it’s okay for you to use the bathroom You don’t have to ask. The extra politeness is appreciated, but not necessary. Just excuse yourself is all.
I can’t even think of a reason why I wouldn’t let a guest in my apartment use the bathroom. “You’re welcome in my apartment, but let’s keep it to the living room, bedroom, and kitchen areas. No bathroom for you!”
I think all you need to do when you visit someone’s home (if the bathroom isn’t within striking distance) is to just ask, “Where is the bathroom?” Then the host will say, “Over there.” And you say “Thanks.”