We can't forgo certain sciences or subjects because it makes somebody uncomfortable. Like any other job, you put-up with things that you may not necessarily agree with, or you move on to another career. The notion that education should make adjustments to its curriculum to appease personal beliefs is reactionary. I honestly don't care what anyone believes. They're entitled to express or feel whatever they like; I don't, however, agree with anyone using those beliefs to deny younger generations a right to a quality education. There should never, ever be any such thing as forbidden knowledge.
If parents aren't comfortable with their children being exposed to certain ideas, there are a couple of things I think they really need to consider:
1.) It's outright censorship. If you're going to shield your children from life, then at least acknowledge what you are doing - that is, that you are literally indoctrinating them with no respect to their personal will. If you can acknowledge this and still consider yourself a good / fair parent, then please stop procreating.
2.) If someone is still insistent on sheltering their children, then send them to a private school that shares your values. Alternatively, you can consider home-schooling. Don't encroach on the rest of society by trying to brute-force your values into public education. Your beliefs aren't considered suitable for conventional education for a very good reason: They're
pseudoscience. Until you can establish the existence of God via the scientific method, don't expect it to appear in textbooks. Same goes for any notion that homosexuality is wrong, is a personal choice, is detrimental, etc.