I'm sure the mother was facing many issues. Working at a company with a sexist environment, her male juniors would get promoted despite being less capable, being constantly shamed about being a single mother. Heck, maybe even the neighbours would gossip about her.
Although, I'm pretty sure the manga won't go in depth of this topic since this is about a teenage girl exploring her sexuality.
The mother being adverse to men is just a plot device.
Edit: I went back to reading that part since I'm an awful speedreader. She may not be working in the stereotype desk job as I imagined, so that's not where her issue with men came from (or else she wouldn't tell her daughter to work at a big company). She either dropped out of university or entered the workforce directly after high school to support her sweetheart's dream, who then walked out of her and her child's life.