Odds are her mom is just, well, a giant prude. Some people are like that. Rather than adequately teaching their children about sexuality, they just make it taboo and repress it. It tends to backfire spectacularly later.
There's also those who make it a religious thing, but they tend to shoehorn "god" as a surrogate authority figure into everything else, not just sexuality. I didn't think that would be a common thing in a place like Japan though.
My theory is that in both cases, deep down, the parent is just too embarrassed to deal with it lol.