'Cause in Texas your wife would shoot you and be no-billed by the grand jury?
Because your neighbors would shun you and businesses refuse to serve you were it to become known?
Because you'd be excommunicated by your church?
Seriously, if under the age of consent, it violates the child's rights as they're not capable of consenting, if over the age of consent, because the state has a compelling interest in preventing the types of disabilities prevalent in births from incestual sexual contacts.
Birth control covers the birth problems. 16 is age of consent.