The simplest and most accurate answer is a dilution of the two historical stereotypes you invoke. Scholars have generally come to agree that while women did enter the workforce in record numbers, even within the workplace, gendered divisions of labor and sexist management perpetuated feminine gendered stereotypes; women were many times more likely to work in a canning factory (because being involved in food production is vaguely domestic) than to be actual riveters in car-converted-to-airplane factories, as Rosie was. [See Ruth Milkman, Gender at Work: The Dynamics of Job Segregation by Sex during World War II (Urbana: University of Illinois Press, 1987).] The unions never warmed to female workers, and consistently fought against their inclusion, let alone their equal pay, because it endangered male prerogatives (they had families depending on them - women, were only doing it for pocket money and to kill time, right?!? /sarcasm)

On the flipside, more women continued working after the war than you might suppose, the largest demographic of such women being actually older, married women, rather than single young ones. [For how a surprisingly large number of women struggled with balancing work and home life, individual aspirations and family duties, see Joanne Meyerowitz, “Beyond the Feminine Mystique: A Reassessment of Postwar Mass Culture, 1946- 1958,” The Journal of American History 79, no. 4 (March 1, 1993): 1455–82, doi:10.2307/2080212.]

Still, there was a huge conservative cultural backlash in the postwar world attributable to many more plausible things than 'eek, women got out of the house!'. Most important to remember is the Cold War context. Americans were seeking to simultaneously define and perform traditional red-blooded Americanism in defiance of the Communist threat, and containment was the name of the game, in both international policy and domestic ideology. [Elaine Tyler May, Homeward Bound: American Families in the Cold War Era (New York: Basic Books, 1988).]

Hope this helps, sorry to be a bit light on statistics, etc. off the top of my head! Hopefully someone else can help sketch out angles I haven't thought of, or feel free to answer follow-up questions!