it's pretty insane since prior to 2005, texas allowed first cousins to marry. they can't go back and charge people who legally married in the state of texas with a crime after the fact. (ex post de facto?)
it's also insane because if you'll remember a few years back, sodomy laws were challenged in texas and taken off the books for the simple fact that it was an unconstitutional violation of privacy to legislate was takes place between consenting adults within the privacy of their own home.
from an article about the court rulings on lawrence v. texas: (https://www.britannica.com/topic/Lawrence-v-Texas)
Liberty protects the person from unwarranted government intrusions into a dwelling or other private places. In our tradition the State is not omnipresent in the home. And there are other spheres of our lives and existence, outside the home, where the State should not be a dominant presence. Freedom extends beyond spatial bounds. Liberty presumes an autonomy of self that includes freedom of thought, belief, expression, and certain intimate conduct. The instant case involves liberty of the person both in its spatial and more transcendent dimensions.
He (Justice Anthony Kennedy) questioned the way in which Justice Byron White, who authored the majority opinion in Bowers, (referencing Bowers v. Hardwick, a 1986 case in Geogia) had framed the central issue of the case—as whether the Constitution “confers a fundamental right upon homosexuals to engage in sodomy”—and asserted that White’s formulation “discloses the Court’s own failure to appreciate the extent of the liberty at stake.” The very wording “demeans the claim of the individual put forward, just as it would demean a married couple were it to be said marriage is simply about the right to have sexual intercourse,” he wrote. (clarification and emphasis mine)
It seems to me that any cousin couple who are legally wed either in texas prior to 2005, or in another state where such marriage is recognized as legal, could find some attorney that is smart enough to use the Lawrence V. Texas case as precedent, since the US Supreme Court ruled that texas could not invade the privacy of a couple in their bedroom.
now... let's move on to the actual statutes in texas. i must point out that i am NOT an attorney and that any commentary i provide on the following is just my personal uneducated grasp of what it's trying to say. i'll put my commentary in red. the entire family code can be found at https://statutes.capitol.texas.gov/Docs/SDocs/FAMILYCODE.pdf
SUBCHAPTER B. PUBLIC POLICY Sec. 1.101. EVERY MARRIAGE PRESUMED VALID. In order to promote the public health and welfare and to provide the necessary records, this code specifies detailed rules to be followed in establishing the marriage relationship. However, in order to provide stability for those entering into the marriage relationship in good faith and to provide for an orderly determination of parentage and security for the children of the relationship, it is the policy of this state to preserve and uphold each marriage against claims of invalidity unless a strong reason exists for holding the marriage void or voidable. Therefore, every marriage entered into in this state is presumed to Statute text rendered on: 9/7/2018 - 1 - be valid unless expressly made void by Chapter 6 or unless expressly made voidable by Chapter 6 and annulled as provided by that chapter. Added by Acts 1997, 75th Leg., ch. 7, Sec. 1, eff. April 17, 1997.
Sec. 1.103. PERSONS MARRIED ELSEWHERE. The law of this state applies to persons married elsewhere who are domiciled in this state. Added by Acts 1997, 75th Leg., ch. 7, Sec. 1, eff. April 17, 1997. (this little clause is what concerns me. now let's go to chapter 6.)
SUBCHAPTER C. DECLARING A MARRIAGE VOID Sec. 6.201. CONSANGUINITY. A marriage is void if one party to the marriage is related to the other as: (1) an ancestor or descendant, by blood or adoption; (2) a brother or sister, of the whole or half blood or by adoption; (3) a parent's brother or sister, of the whole or half blood or by adoption; or (4) a son or daughter of a brother or sister, of the whole or half blood or by adoption. Added by Acts 1997, 75th Leg., ch. 7, Sec. 1, eff. April 17, 1997. (so as we know, cousins can not marry.
Sec. 6.307. JURISDICTION TO DECLARE MARRIAGE VOID. (a) Either party to a marriage made void by this chapter may sue to have the marriage declared void, or the court may declare the marriage void in a collateral proceeding. (b) The court may declare a marriage void only if: (1) the purported marriage was contracted in this state; or (2) either party is domiciled in this state. (c) A suit to have a marriage declared void is a suit in rem, affecting the status of the parties to the purported marriage. Added by Acts 1997, 75th Leg., ch. 7, Sec. 1, eff. April 17, 1997. (now maybe i'm wrong, but it sounds to me as though the state of texas is required to acknowledge as valid any marriage that (at least legally) took place somewhere else, assuming the couple did not live in texas at the time they married elsewhere.) (i emphasised what is in bold.)
So... MY understanding (which is worthless in the big scheme of things) is that a couple who resided and married where cousins could legally marry are safe from being declared void by the state of texas. and it is also my understanding (also worthless in the big scheme of things) is that a judge can not peer into the private bedroom of a couple and declare their sexual conduct as illegal because that was fought (in the US supreme court) in lawrence v. texas.
at the very least, if any couple ever faces legal action in texas for having sex with the cousin they are married to (or living with, since there is another whole section on the validity of common law marriage), find a lawyer who will be willing to dissect all of the above in front of a judge, and who is willing to take it all the way to the supreme court if necessary.
i honestly don't see texas ever reversing it's relatively recent ban on cousin marriage UNLESS it is challenged in the court.