I've asserted several times in this series that bars were, traditionally, male spaces. It wasn't until checking Christine Sismondo's phenomenal history America Walks Into a Bar out from my local library that I found out this was not just an informal taboo: in the decades after Prohibition, many bars explicitly banned women, or banned them from visiting during certain hours.
There were a few reasons for this, depending on the region and the bar: first, during World War II, as was the case in many other fields, women went into traditionally male occupations, including bartending (in some cases forming barmaids' unions). When men came back from the war, they formed their own, all-male unions to muscle female bartenders out. But bars did employ women during the postwar era – just not to pour drinks. Instead, "B-girls" employed by the bar would show up, pretending to be nurses or secretaries on their way home from work, and charm the male clientele into buying them drink after drink. After several drinks, the woman in question – usually called a "B-girl" – would disappear, leaving her companion with an artificially inflated bar tab: he'd be charged for cocktails even as the in-the-know bartenders had been pouring one glass of juice, soda pop or iced tea after another.
The ensuing moral panic (which focused on protecting the male victims, and didn't concern itself one way or another with the women involved in the work) had the result that many bars banned women from visiting, or just from visiting during certain hours. And, of course, there were the bars that had never opened their doors to women in the first place, or just refused to serve unaccompanied women.