This isn't straight girl behavior. Why is everyone calling her a straight girl?
She basically communicated without saying outright that she was into Shigisawa when they were working together. It's not like this is (just) acting out over her husband being shitty.
At this point, she's tried being married to a man and she's clearly decided it's too much for her. To her, the possibility of getting together with an old crush must be so tempting that she can't hold back while drunk after a bad day.
Because She IS straight, for now at least. From what the author show us and the way Shigisawa saw her, yes She is straight. And she even confirmed it in this chapter. Her main goal in life was to marry a man, have babies, and that's it. At chapter 2, Gotou is still straight, maybe she has some curiosity for women (Shigisawa) by what we saw at the end of the chapter, but for now she's straight. Also, straight girl behavior ? Is that a thing. Everyone have their own behavior, being straight or not.
And when they were working together she was just fascinated and impressed by Shigisawa because she was capable at work and could do anything by herself unlike her. Again she said it in this chapter. She was so impressed because she could never have imagined one day to see a woman being so capable and reliable at work like Shigisawa. Her joining the company was just to find a man to marry and I think, She told herself that it was the same case for all other women in the company. Love and admiration are two different things. The only person who have a BIG crush on someone here is Shigisawa. And I'm pretty sure Gotou know that Shigisawa loves her, and maybe that she is gay.
Of course, at the end of the story she will be with Shigisawa, so she will be a lesbian or at least bi/pan. Literally this story it's about her discovering her new sexuality and Shigisawa to fall in love again and trust her lover to not backstab her AGAIN to go live the Japanese straight dream.
Also, the scene when the two of them are drinking and start to be really close.... This kind of scene needs to die, It's so cliché. Using alcohol to be close with someone can lead to some really messy and serious shits after.