They're not legally married; polygamy is illegal in Texas.
The age of consent is 17 in TX. It's totally irrelevant who did what in the past and where around the world it happened. What matters is what has happened now in the state of Texas, and sex with a minor girl under most circumstances is statutory rape.
If you don't like that, go have the law changed. Until then, it's the law and people have to abide by it. Or move where the law suits them better.
Since they are not legally married (civil) then they cannot be committing illegal polygamy since the definition of polygamy is MARRIED to more than one spouse.
Polygamy is illegal everywhere. Shacking up with multiple partners is OK though. IF the partners are of legal age to consent.
However polygamy may not apply if she is his first and/or only wife and they were legally married. A big IF to be sure. Even polygamists can have one legal wife. Since he's only supposed to be 22 (and why say that, since saying 21 would make all the sex legal, (3 years or less separation, and the younger at least 14). marriage aside, or at least non prosecutable. If these folks are as devious as some here say, surely they'd have thought of that.)
It’s rather ironic, isn’t it, that a group who wants to be ignored by the law persists in ignoring the law?