Historically, all premarital sex was illegal, or at least against public policy. The quirk is not that it is illegal for minors to have premarital sex. The quirk is that it is legal for adults to do so.
The law will catch up to reality eventually.
Not only weird but it often creates very dangerous situations for the young women entering into the marriages. Women married before 18 are substantially more likely to suffer domestic abuse. Its a legal loophole for abusers and stupid law.
https://www.unchainedatlast.org/child-marriage-devastating-consequences/