Why do many parents prevent children from being exposed to anything sex-related? We remove children from anything that is sex related, including talks and photos. When I think about it, it seems deeply and morally wrong to not do it.

When I first thought about that, I thought that it might be because we don't want to encourage them to become sexual. But we also don't want them to be violent, and no one thinks it's wrong to send a 5 year old to a martial arts class. For any other aspect of life, we let them, to some degree, experience.

That made me think that it might be something more than cultural.  Of course the definition of "child" may vary depending on the society,  but not the moral code of that ruling.
If I were to guess, the reason is probably something to the effect of: because children would not understand since they lack the same (or same level of) sexual urges that adults have. Another thought that comes to mind is maybe parents are afraid (or even ashamed) of their children seeing them (the parents) acting essentially like non-human animals. Sexual intercourse activities are usually very animalistic compared to most of the things we do in everyday life of modern society. For a similar reason, many parents probably would not want their kids seeing the parents drunk or otherwise acting uncivilised. In a way, it is like the parents are trying to hide from the kids the fact that adults are in many ways just big babies who are good at hiding it. The idea that humans are supposed to be civilised and clean is somewhat unnatural, yet common culture seems to view this idea as highly important.

On the topic of war and violence, my guess as to why these are often considered okay for children to see is because awareness of danger and the threat of violence are necessary even for a child, while knowledge of sex and sexuality really is not that essential at a young age.