If teenage girls are the pinnacle of female attractiveness, why

Because feminists and white knights.

Because the majority of people living in this society are not teenagers, and therefore feel bad that the best years of their life are long over.

It's not in anyone's best interest. Men would feel worse about having no or next to no chance to get teenage girls, women would feel bad that even at 20 they're already on a downhill slope. Still, with the amount of delusion in the world regarding this is stupidly high, especially among women.

Because if men were allowed to marry younger women, they would be able to become wives and mothers earlier in life.
It would create stable family units, with women being able to raise a family, and men being able to provide for them, fulfilment for everybody.

Mr Goldberg hates stable families and fulfilment.

If it is unacceptable for men to marry younger women, Mr Goldberg gets another few years to fill her head with his subversive poison.
Another few years to convince her that marriage and family is wrong and tantamount to slavery.
Another few years to push her towards promiscuity and degeneracy, to make her a less attractive partner.

Because even if that were true, teenagers are not mature enough to have relationships with adults.