They do have the right to withdraw their child from sex-ed classes if they wish.
I think the nondiscrimination part of the curriculum is mandatory. I believe that parents should not be able to remove their kids out of public school for nondiscrimination education, because:
1) It's the law of the land. Discrimination based on racial origin, religion, sexual orientation, disabilities, etc. is illegal.
2) Whatever imagined harm may come to kids of conservative parents being exposed to notions of respect pales in comparison with the actual harm experienced by bullied students every single day.
Bullies assault those they perceive as inferior. All students have the right to a safe and positive learning environment. To protect all kids, tolerance and respect for difference must be taught to everyone. It's part of the anti-bullying law passed in 2012.