OK, let's say you're right, and "nobody is brainwashing kids to be gay".
So explain, what exactly those teachers are doing?
Helping them to understand each other and themselves... this isn't complicated.
Now, for way of comparison, I went to 12 years of Catholic Schools in the 1970's. I'm straight, but I could imagine what kind of horror one of my classmates who was gay would have gone through listening to the constant drumbeat of how they were going to Hell.
Now, yes, a 3rd grader isn't going to have any idea of his sexuality yet, but letting him know gay people are a thing that exist and that there's nothing really wrong with it is going to prevent a LOT of problems down the road.
And letting the straight kid know that hate is unacceptable at an early age is also a good idea.
Now, final point, something that is only tangetally related. I have a six year old great nephew. When my brother took him fishing (he loves to fish since he was 3) and they were releasing the bait fish at the end of the trip, he asked, "These aren't invasive species, are they?"
My response was, "He's six! How does he know what an invasive species is?" Obviously, he picked up the concept from somewhere. Probably TV. He likes the nature shows.
School is only one institution that kids are going to learn things from, but they are the ones who have the responsibility to put things in the proper context.