Shouldn’t be that unpopular, really. Under those laws, a 16 year old can marry an 18 year old no problem. But, if things aren’t so great a year later (and let’s be honest, we all know people who’ve had those marriages), the now 17 year old needs permission from their adult spouse (parents don’t count anymore, since marriage also emancipates them) to retain an attorney or file for divorce.
Giving the maximum benefit of doubt here and assuming that Romeo and Juliette laws are an honest attempt not to saddle young lovers with a lifetime offender registration, the marriage component of them should be scrapped.