The first-ever female US Senator from Wyoming has come under fire for comments made at a university commencement speech – about gender.

Republican Cynthia Lummis, 67, a conservative stalwart, was booed by students and ceremony attendees on Saturday during her speech to graduates of the University of Wyoming, her alma mater; she earned degrees in animal science, biology and law.

“Even fundamental scientific truths, such as the existence of two sexes, male and female, are subject to challenge these days,” Sen Lummis said to jeers that rumbled for 30 seconds and continued as she eventually resumed speaking, smiling at the crowd from under her cap and tassle.

“And I challenge those of you...,” she said, still grinning as the boos went on and she interjected: “I’m not making a comment on the fact that there are people who transition between sexes.”

The grandmother-of-two has since apologised for her comments, according to Wyoming outlet Oil City News.

“It was never my intention to make anyone feel un-welcomed or disrespected,” she said in a statement. “My reference to the existence of two sexes was intended to highlight the times in which we find ourselves, times in which the metric of biological sex is under debate with potential implications for the shared Wyoming value of equality.”

University of Wyoming’s president, Ed Seidel, released a statement the day after Sen Lummis’ speech.

“One of our speakers made remarks regarding biological sex that many on campus take issue with,” he said. “While we respect the right of all to express their views, from students to elected officials, we unequivocally state that UW is an institution that supports and celebrates its diverse communities that collectively make us the wonderful place that we are.”

Wyoming, a consistently red state, was the site of the tragic murder of openly gay UW student Matthew Shepard almost 22 years ago – an incident which many credit as a turning point in the battle for LGBT+ rights.

In March, the state Senate passed legislation that would ban transgender women and girls from competing on teams matching their gender identity, regardless of their assigned gender at birth. The legislation will continue on to the state House of Representatives.

The ACLU of Wyoming said the legislation was “not about leveling the playing field for student athletes”.

“It’s about erasing and excluding trans people from participation in all aspects of public life,” ACLU advocacy director Antonio Serrano said after the state senate passed the measure. “Extreme policies such as Senate File 51 are out of step with prevailing international and national norms of athletic competition. Inclusive teams that support all athletes and encourage participation should be the standard for all school sports.”