Whilst it might well be the case that boys at Mack Beck's HS and in Texas HS wrestling generally didn't want to wrestle with or against female opponents at the time Mack Beggs was competing in HS wrestling, I don't think the boys themselves ever had much say in the matter. The Texas school school officials who serve on the University Interscholastic League, the governing body that makes the rules for HS sports in Texas, are the ones who made the decision that Mack Beggs had to compete in the division of high school wrestling that Texas had created expressly for members of her sex.

Texas was actually on the cutting-edge and way out ahead of most other states when it started an offiicially-sanctioned high school wrestling program for girls on par with the high school wrestling it offered for boys back in 1999. Texas was only the second state in the US to do this. Hawaii started offering wrestling for HS girls on par with boys one year earlier, in 1998.

At the time Mack Beggs was wrestling in HS, only six states in the USA altogether had separate full-fledged, officially sanctioned wrestling programs for high school girls.

As of the 2017-18 season, there are six high school state associations which have officially sanctioned state high school championships for girls. The states with girls wrestling, with the year of their sanction, are Alaska (2014), California (2011), Hawaii (1998), Tennessee (2015), Texas (1999) and Washington (2007).

https://nwhof.org/news/high-school-girls-wrestling-continues-rapid-growth

My understanding is that ever since Texas began a separate full-fledged wrestling program for high school girls back in 1999, the rule in the Lone Star state was always that:

(g) Boys may not wrestle against girls, and vice versa. This prohibition is only applicable when the contest is held in Texas or in any other state that sponsors wrestling programs for both boys and girls

https://www.uiltexas.org/policy/constitution/general/nondiscrimination

The only issue that was unclear when Beggs started HS wrestling was exactly what criteria schools should use to determine which students are on the boys' teams, and which are on the girl's teams. In 2016, Texas decided to base it on the sex stated on students' birth cerificates.

BTW, in the states without separate HS wrestling for girls, it's typically been the case that girls were/are allowed to try out for boys' teams and they've wrestled boys in interscholastic meets if they made the team.

But today, most high school girls in the US don't have to go out for boys' teams any more because a total of 46 states now offer separate, full-fledged officially-sanctioned wrestling programs for high school girls. 40 of these programs have been started since 2018; 27 of them have been added just since 2020.

https://wrestlelikeagirl.org/hsseau

More on the Texas rules back when Mack Beggs was in HS wrestling:

March 3, 2016

Texas Public School Superintendents have overwhelmingly approved a new rule for the UIL - the University Interscholastic League - that requires students to play on the team that matches the gender listed on their birth certificate. The rule is being criticized by LGBT rights advocates.

95 percent of public school superintendents in Texas who voted on the rule supported it. It requires Texas' public school athletes in middle and high school to play on teams reflected their birth certificates. Thus a transgender transitioning student would not be able to play on the team that they identify with.

Kate Hector is the spokesperson for the UIL. She explained the rule on TPR’s The Source on Thursday. “This is really placing into rule which is already in practice. So if there were any questions that coaches or athletic directors had they would call the UIL and this is the practice that we would advise and has been for a while and this new rule is just placing it into policy.”

https://www.tpr.org/community/2016-03-03/the-source-new-rule-doesnt-recognize-transgender-teens

May 30, 2016,

last week, Texas along with 10 other states sued the Obama Administration over a new directive about transgender students' access to bathrooms in public schools.

And in two months, a new Texas policy will make it nearly impossible for transgender athletes in the state to compete in high school sports.

Earlier this year, Texas school superintendents voted in favor of a rule change that requires all student athletes to compete under the gender listed on their birth certificate.

Kate Hector, spokeswoman for the University Interscholastic League, which governs Texas public schools' athletics rules and policies, says the measure was necessary and needed to be put in writing.

"The UIL recently went through a complete review of all of our rules," says Hector. "And this rule is something that's been in place for a while and practiced, but it was not written into policy. So it was really correcting that."

But Lou Weaver of Equality Texas says the rule is discriminatory and violates Title IX — a federal law that prohibits discrimination on the basis of sex in all education programs operated by those institutions who receive federal funding.

https://www.houstonpublicmedia.org/articles/news/2016/05/30/154564/controversial-rule-to-exclude-transgender-athletes-in-texas-public-schools/

What do UIL rules say about transgender athletes?

The UIL rule (Section 360 of UIL Constitution and Contest Rules), which is set by representatives of member school districts across Texas, states that gender will be determined based on a student’s birth certificate for the purpose of UIL participation and competition.

How was the birth certificate rule established?

After approval by the Legislative Council, the rule regarding birth certificates was placed, along with other UIL rules, on a referendum ballot so all UIL-member school superintendents could vote on this addition to the UIL Constitution and Contest Rules. On the referendum ballot, 95 percent of UIL-member school superintendents voted in favor of this item. The results of the ballot can be found here: https://www.uiltexas.org/files/policy/2016_spring_referendum_ballot.pdf (ballot item 3)

The Legislative Council and the Commissioner of Education approved this rule change before it took effect on August 1, 2016.

https://www.uiltexas.org/press-releases/detail/uil-statement-regarding-2017-wrestling-state-tournament