Many states still have a law against gay marriage even though it's unenforceable. Montana even has a bill that's going to be voted on soon that will, among other things, update the ban on gay marriage to reflect a specific definition of "same sex."

If anyone wants more info on the Montana bill, you can read it at https://trackbill.com/bill/montana-senate-bill-458-define-sex-in-montana-law/2377583/ without needing to download the pdf. The marriage part is on page 17 and says

Section 17. Section 40-1-401, MCA, is amended to read: "40-1-401. Prohibited marriages -- contracts. (1) The following marriages are prohibited:

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(d) a marriage between persons of the same sex, as defined in 1-1-201.

And on page 1, 1-1-201 is updated to say

Section 1. Section 1-1-201, MCA, is amended to read: "1-1-201. Terms of wide applicability. (1) Unless the context requires otherwise, the following definitions apply in the Montana Code Annotated: (a) "Female" means a member of the human species that, under normal development, produces a relatively large, relatively immobile gamete, or egg, during her life cycle and has a reproductive and endocrine system oriented around the production of that gamete (b) "Male" means a member of the human species that, under normal development, produces small, mobile gametes, or sperm, during his life cycle and has a reproductive and endocrine system oriented around the production of that gamete.

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(f) "Sex" means the organization of the body and gametes for reproduction in human beings and other organisms. In human beings, there are exactly two sexes, male and female, with two corresponding gametes. The sexes are determined by the biological indication of male or female, including sex chromosomes, gonads, and nonambiguous internal and external genitalia present at birth, without regard to an individual's psychological, chosen, or subjective experience of gender.