Real answer: because sex takes over everything, while Nazis only wish they could. People come up with all these complicated hypotheses about puritanism and shriveled old men controlling everything not wanting anyone to have fun - if you think those types of reasons would overrule people's greed for more traffic and more money, you have too rosy a view of humanity.

The simple truth is that once you allow sexual content on a platform, the majority of the content you'll be serving henceforth will be sexual. Advertisers don't like that, most parents don't want that, and your legal team and your PR/marketing team aren't gonna like the thorny and potentially brand-ruining issues this is going to bring in.