To me, safe means I can browse it at work and if I get caught by my boss he's not going to think I'm browsing for porn. Or if I browse Safebooru at a public library, people may get annoyed at me but they're not going to complain to the librarian that someone's looking at pictures of girls.
So I actually think your definition of safe is too lax. I think safe should exclude lingerie, almost all nudity, two piece swimsuits, and large visible breasts/ass. It's not really a matter of whether you think an image is sexualized, it's rather if some random person, at a glance, would consider it inappropriate to show that image in public in front of everyone (including children).
But I don't think hardly anyone agrees with me that safe should be taken that far. I still would not feel all that comfortable browsing Safebooru (as it is now) at work because the occasional buxom bikini girl shows up, but it's considerably safer than Danbooru itself.