The consititution allows states to set many laws, and to safeguard rights according to those laws. And in Texas it is illegal for girls under 16 to marry. This law trumps religious freedom. There are various statutory laws regarding sex with minors (these often times catch minor young men having consentual sex with minor young women--the system isn't perfect).

The thorny definition of "religion" has crossed paths with the socially unacceptable behavior of "cults" many times in the past. Cults, in their general interpretation of 'self-serving make-believe religion' usually lose.

SRS