Ancient people lived in a pristine natural environment,
Because they didn't have the technology to "tame" nature. I watched a documentary that said Greeks really didn't like nature that much. The city was the place to be.
were not threatened by overpopulation,
Pretty sure that's not true. Poor people having lots of kids and cramped apartments was a thing during the Roman Empire. Why do you think bread and circuses was a thing?
tides of lethal sexual diseases
They had STDs in Roman times.
and incurable cancers,
Pretty sure they had cancer back then too. They probably just didn't know what it was so called it something else. Like consumption.
Gender confusion
I'm pretty sure you're wrong in regards to the Romans. I believe India had a concept of a "third gender".
Inability to survive the presence of basic food staples like wheat and milk,
Because they died off young if they couldn't consume those things. That's why you didn't hear of it. Experimenting with diet for health reasons was probably still a thing though. Epicurus thought a life in moderation was the best lived life.