Exactly, all I know is that there IS a rule saying you cannot post links to you, or a friend's FA account.
While FA isn't technically a p*** site, it is commonly used as one! But true, like you said, you would need an account, and to be +18 or older in order to view such art, or "pornography", as we would put it. That, or post any porn to the site!
DA is practically the only site as well as I'm aware of you can post links to. Any other site that is NOT dA related, and includes porn, be the website consisting of mainly porn or other pictures that are relevantly close to it, may NOT be given a link to!
If you can't post a link to that exact rule, then i'm just going to assume this rule your talking about doesn't exist.
And just because the site is used as a porn site, doesn't make it one. Not only does it not advertise itself as one, you also need an account and to fit a mature age and to click a few conformation buttons saying you want to view the mature content.
And again, post a link to that exact rule, other wise, i'm just going to assume it's a myth.