Montana may sign a law defining sex as strictly "male" or "female," which would make it the fourth U.S. state to pass such legislation.

Governor Greg Gianforte, who has already authorized a bill to ban gender-affirming treatments for transgender minors this year, has until Sunday to sign or veto the new legislation, Senate Bill 458, or return it to the legislature for amendments.

He could join fellow Republican governors in Tennessee and North Dakota, who signed similar laws this year. In Kansas, the Republican-dominated legislature overrode the veto of Democratic Governor Laura Kelly to enact its law.

Republican sponsors of the bills say they are not meant to discriminate against anyone, stressing that discrimination is already illegal.

"It seems 20 years ago nobody needed a definition of sex because everyone understood what it meant. But now there is a discrepancy about, 'Is sex gender and can I change it?' But sex you can't change. Sex is just a fact," said Montana state Senator Carl Glimm, who introduced SB 458.

Major medical and psychological associations endorse gender-affirming care and say transgender identities should be respected, while conservative groups claim that children are too easily allowed to transition.

The Human Rights Campaign (HRC), the largest advocacy group for lesbian, gay, bisexual, transgender and queer people in the United States, has labeled the laws as "erasure acts" aimed at forcing queer people back into the closet.

"We fear that these LGBTQ+ erasure acts could be the next type of anti-LGBTQ+ legislation to sweep the country," HRC President Kelley Robinson told reporters this week.

These new laws define virtually everyone as either "male" or "female," a determination made at birth based on anatomy and genetics that cannot change. The Montana law says sexes are determined "without regard to an individual's psychological, behavioral, social, chosen, or subjective experience of gender." The Tennessee and Kansas laws make no allowance for intersex people or individuals born with ambiguous genitalia or irregular chromosomes.

While researchers say sex generally refers to physiological characteristics and gender is more a social construct, when it comes to federal civil rights law, they are essentially the same. In the landmark 2020 decision in Bostock v. Clayton County, the U.S. Supreme Court found that discrimination protections based on "sex" also applied to sexual orientation and gender identity.

"By defining sex so narrowly, you are excluding LGBTQ people from bringing claims in state court based on discrimination on the basis of sex," said Sarah Warbelow, HRC's legal director.

The laws also stand to limit nontransgender people who have a discrimination claim based on sex stereotyping, Warbelow said.

Tennessee state Representative Gino Bulso, who sponsored the bill that Governor Bill Lee signed on Wednesday, said arguments surrounding the Bostock decision or federal anti-discrimination laws never came up in the debate.

"This just deals with sex. This bill is not going to affect the term gender, wherever it may happen to be used," Bulso said.

So far this year, Republican legislators in statehouses across the country have introduced more than 500 bills affecting LGBTQ rights with a particular focus on transgender people. Around 50 have become law.