Is it appropriate for a woman to use the men's bathroom if it is a single bathroom?

Restaurants and other locations often have two one-room bathrooms (i.e., no stalls, just a small room with one toilet and sink). I’ve never understood why they would put a “women’s” sign on one and a “men’s” sign on the other if it’s a single room without stalls, rather then just have two bathrooms that could be used by either women or men (some restaurants do in fact do this, and have a sign that shows the bathroom could be used by either women or men).

The question is: If the women’s one-room bathroom is occupied, is it inappropriate for a woman to just use the empty one-room men’s bathroom, rather than wait?