Very good idea. The perfect solution but neither side really wants it. Why? Because what both sides really want is for OTHER people's kids to believe what they want their OWN kids to believe. Liberals want Christian kids to be taught that homosexuality is normal. Christians want liberals kids to be taught that it is wrong. The solution for both sides is to teach your own kids your values and hope for the best. But that's hard work and who has time? Let the schools do it.
There is one big difference. The entire history of the United States agrees with the parents who oppose the legitimization of homosexuality.
It is the left who has done an end run around the Constitution by using activist judges to force the left's view into the Public Schools. There was no vote taken, the parents wishes were ignored. It is the left who has used the Public Schools as a way to indoctrinate children into a worldview in opposition to the overwhelming majority of parents.
You will also find that this is why the left fights so hard to oppose any move to defund Big Government Schooling and begin the process of privitization.
Because liberals know very well that the Public School System has (by liberal judicial activism) become the left's only hope of undermining what most children are taught at home and at church, they use every tactic available to keep that control from parents and communities. This is also why the left-wing press so strongly opposes privatization and home-schooling.