So I see that the guidelines are all spelled out quite well in the link shown, in particular:

Linking to a site which meets any of the following criteria in a post is prohibited:

    Advertises, suggests or even hints that minors may be discussed or portrayed on, or contacted through, the site.
    Contains Escort Reviews (Ex.: ECCIE, Erotic Monkey);
    Has Pop-Ups (Ex.: Porn and cam sites);
    Is likely to contain malware;
    Requires login (registration) for viewing the contents of the site (Ex.: Facebook);
    Is absolutely irrelevant to the subject;
    Belongs to a private business or which offers specialty services (Ex.: Ashley Madison)

Cheers!