I don't get it. I understand the part about not wanting to marry but why no hookups?
Oral contraceptives were illegal in Japan until 1999 and many women shun them because of concern over side effects and thus prefer contraceptive methods such as the condom that don't require messing with their biochemistry. Abortion is still technically illegal, tho it can be obtained from "approved doctors" under conditions so liberal that pretty much anyone can get them unless you are of perfect wealth and health. Altho if you are married, then for that class of abortions we'd generally class as elective abortions, you need your husband to agree.
So while family planning services are available in Japan, they aren't something that people want to avail themselves of. They see it as simpler to just say no.
Japan is an odd mix of prudery and openness when it comes to sex.