Seems overly protective, and possibly a bit ridiculous depending on your sensibilities, if thought through.

By similar logic supermarkets should not carry alcohol or tobacco, theaters cannot show 18+ movies (even non-explicit ones), and entire parts of some cities need to be redone because of their red-light districts, because there are some at central locations a kid could reasonably stumble into.

I think just restricting access to this stuff, being discreet about it, and maybe limiting advertisement, is enough. I've lived somewhere with a pretty plainly visible red-light district close to the central train station, yet most people don't even realize it is there. I'd hope something similar could be accomplished for Steam as well.

Finally, at the end of the day parents gotta parent.