Google said it can take many months for Google to reclassify a web site that was classified as being adult, to not being adult anymore. Google’s John Mueller said on Twitter “these things take quite a bit of time to be reviewed and re-evaluated by our algorithms, usually it’s on the order of months.”
He added that timing can vary because there can be many factors. John wrote, “tt depends quite a bit on what’s involved.” “Some things can be automatically recognized when they’re first seen, other kinds of changes need to be updated in algorithms, which can take quite a bit longer,” he added.
Why? I believe the SafeSearch filter is on a URL by URL basis and in some cases are site wide domains.
There is this form you can use to communicate to Google that “your content is mistakenly being filtered by our [Google’s] algorithms, and you’re sure you’re not hosting adult content on your site.”
In some cases, it might require Google to update their filters and algorithms. In other cases, it might require a recrawl. One takes longer than the other, obviously.
Forum discussion at Twitter.