Because even with the laws in place currently, we have sophists making excuses for the FLDS. Making their behavior very clearly illegal makes the prosecution that much easier. If the law had not been changed, rest assured that there would be a legion of Freepers defending their right to marry 14 year olds "spiritually" as they could marry them legally with a marriage certificate and who needs the government anyway? Surely you can agree that that defense that would have been advanced if the law had not been changed. Yes?
What behavior? Polygamy was already illegal. Non consensual sex with anyone was already illegal. Absent lawful marriage sex with someone 14-16 was already illegal, unless the partners were 3 or less years apart. All the law did was ban legal marriages for 14 and 15 y/o olds.
If the law had not been changed, rest assured that there would be a legion of Freepers defending their right to marry 14 year olds "spiritually" as they could marry them legally with a marriage certificate and who needs the government anyway? Surely you can agree that that defense that would have been advanced if the law had not been changed. Yes?
Maybe, but it would still only apply to one male one female marriages, not polygamous ones, nor to otherwise legal unions of anyone but 14 and 15 year olds.
The issue of requiring state permission is a separate one. There are plenty of other folks, having nothing to do with polygamy or Mormons of any stripe, who think the government should never have gotten into the business of marriage licenses and such. It is still a relatively new thing even by US standards, let alone if one goes back to the northern European cultures from which our nation grew. Previously all marriages were either "spiritual" that is sanctioned by a church or other religious body, common law, or civil, that is conducted by a civil official (that was fairly uncommon as I understand it, common law was less rare than civil)