It's true that Sayu would definitely have to face that question eventually. I don't think the MC would bring it up either as he's too trusting and not conscientious of her.
While I still hate Gotou, I can see her relevance to the plot. But yeah... none of the heroines are really that good. They all have major flaws. Only one who's flaws I can deal with so far are the Kouhai's.
Gotou(Senpai) - Manipulative and selfish. She liked the fact that the MC liked her, and as soon as he was ready to move on she makes a move on him. She doesn't actually want to be in a relationship with him or else she wouldn't make such an extreme lie and hurt him emotionally like she did when he confessed him.
But she's socially competent and attractive.
Sayu(Runaway) - Young and immature. She ran from her problems, slept with dozens of other men, and doesn't really respect her body that way(she makes the moves on MC at one point) while having intimacy issues. She's very reliant as well. By far the most of a hassle. Dealing with guys that might show up in the future is also a hassle. Literal NTR bait. Honestly... by far the worst choice imho.
But she's young and attractive and actually has feelings for the MC.
Kouhai - Lazy. This is my assumption but she probably wants a normal nuclear family where the wife stays at home and the man works. She puts the least amount of effort possible to finish her work, and does whatever she can so that the object of her affection has to interact with her.
But she actually has feelings for the MC, and is very understanding and supportive.
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The fact that everyone including the MC has glaring imperfections makes them feel more human though. MC's flaws are not nearly as bad though. His main flaw is that he's too accepting. He just accepts the highschool runaway. He just accepts the fact that his Senpai is a virgin who actually loves him (LOL!!!!) He just accepts having to deal with his kouhai who for some reason can't get the hang of work even after a year at the company.
His flaw weirdly enough is what draws the girls to him, but it's still a flaw nonetheless.