It'd been awhile since I read that story, so I refreshed my memory with the link provided in the other thread. After having done so, I'm not so sure that it was directly Queenie and Pals that motivated Sammy to leave his job. Also, there are other things to consider.
Throughout the story Sammy is constantly talking about people as if they are "sheep" and making other comments along the same lines. Reading between the lines about his feelings for Lengel, as well as the job itself, it seems as though Queenie and Pals are only the catalyst, not the true motivation, for Sammy's resignation.
You asked about John Updike using females as sex objects in his stories, and again, there's a slight sticky point here, one the requires you delve into the personal motivation of the character. If Sammy is sexist, he'll see the girls as objects, but that doesn't necessarily speak to Updike himself. Separation between author and content is really key - if an author writes a story about a serial killer, does that make him see all people as objects, simply because his character does? No, certainly not.
So, if Sammy is the sexist, not necessarily Updike, we'd assume that he would not quit his job simply over three young girls - they wouldn't matter enough to him. There must be a further motivation. We can also assume, considering the final line, which has nothing to do with the girls at all, that the internal struggle of Sammy is more economically motivated. Several times he mentions the economic class of the girls, as well as considers his own and Lengel's. We also know it summer, and Sammy seems young enough to be a student since his mother ironed his shirt. Perhaps this is a young man still in highschool who will soon return to class? Or a college kid, home to work for the summer? We can't say, but these details bespeak more of a youthful, economic motivation for Sammy, with the girls only providing the catalyst.
If you're simply writing about Updike's continued use of women throughout his stories as catalysts, perhaps it would behoove you to delve a little deeper into why he might choose them. You're probably correct that Sammy would not have seen three young men as a viable catalyst, but that speaks to Sammy's character, not Updike's. What more likely speaks to Updike's character is his choice to write a story with a character like Sammy, a character who would be motivated by three girls. Once Sammy is created, the only logical choice for Updike, in order to provide the proper catalyst for Sammy, is to write in three girls. If Updike had included Sammy, but then written in three boys, Sammy wouldn't have reacted, and Updike would have no story.
So the real thing to consider, if you want to approach it from a gender issues angle, is why include Sammy? Why not a female cashier?
I remember them but I wasn't sure if they were still around.