Boys and girls who break this Halacha and are boyfriend and girlfriend should not be allowed to marry just like we don’t allow a man who cheats with a married woman to marry her after her divorce. And if they do get married, they should be forced to divorce, just like the other relationship that began without kedusha.
That’s a good idea. Why don’t you tell Hashem that, and maybe next time He creates a world, He’ll put that in the Torah.
(Or tell the chachamim and maybe next time they have an anshei knesses hagdola, they’ll make that gzeirah.)