Responses
Because adults, especially parents doesn't want to encourage teenage sex.
And there's definitely a regulation for publishing company about censorship and rating system. Shoujo manga is usually for girl from elementary to high school. I don't think it's right to show adolescence girl that's in HS sex is a common thing. Especially when they didn't even get sex education to have safe sex.
i dont really get the mindset in these shoujo mangas that theyre too young for sex in HS when its not uncommon irl in Japan...