Sounds like these parents oppose freedom of speech.
I think it's more that they oppose having mandatory participation for certain lesson plans and curriculum content.
"they gathered to oppose Montgomery County Public Schools’ removal of its opt-out option for elementary reading materials that cover gay and transgender topics."
The ability to "opt out" was always the "last line of defense" for parents, so to speak. And while some left-leaning members of academia have been persistently saying "this is just education, not indoctrination", the moves they're making seem to cut against that.
I remember back in the day when I was in high school and some parents didn't want their kids learning about sex ed or evolution, they'd opt them out and have an alternate lesson plan ready or just give them an extra study hall. I'd argue that those topics were far less polarizing and controversial than these ones.
When these conversations started a few years back, the left had more people on their side than not, which each little bit further they move the needle, the more and more people they lose to the "other side" of this particular set of issues.
When it started, they had the overwhelming majority of Democrats, over half of Independents, and even a small percentage of Republicans.
For instance when this was strictly just talking about High School, 2019 polling showed:
About 85 percent of Democrats (as well as 60 percent of Independents) said that high school students should learn about sexual orientation and gender identity. That compares with 37 percent of Republicans who want students to learn about sexual orientation and 32 percent who want them taught about gender identity. (averaging out to about 6-7 in 10 of all Americans in favor)
When they kept moving the line and pushing for younger and younger target audiences, now 2023 polling finds this:
Fewer 50 percent of Democrats supporting the teaching of both transgender rights and same-sex rights in elementary schools, while fewer than 1 in 10 Republicans support teaching those topics in elementary schools. (averaging out to only about 3 in 10 Americans in favor)
They went from positions that were largely popular, to pushing for ones that are quite unpopular.
When they can't even get over 50% of their own party to support something, and then combine that with subsequent rules like "You're not allowed to have any input on this, and when we actually do teach it, you're not allowed to show up at school board meeting to complain about it, else be labelled as a domestic extremist" The backlash was quite predictable, and in the midst of that backlash, schools like this one in the article decided it was a good idea to remove parents' ability to opt their kids out of elementary school content that polling shows is largely unpopular.
Unpopular ideas + "You just have to sit down, keep your mouth shut, and let it happen" = Cultural powder keg
The lesson that can be inferred from the poll results above? High school is where this kind of content should probably be covered and not 4th grade, and if a school is going to try to include it at much younger ages, they should probably not take away the opt out ability of parents.