Well, it can still be tricky. OK, so a link that directs you straight to porn pics is not allowed. What about a link to a porn site that doesn't have any pics displayed on the front page? What about a link to a site that has links to porn sites on it? What about a link to a thread that contains links that go straight to porn pics?
You have to draw the line somewhere, and just saying "no porn links" doesn't usually cut it. Some sites use a three link rule (porn has to be at least three clicks away), but sometimes that would make a perfectly harmless link out-of-bounds according to that rule, so I think it's a bit over-the-top.
Anyway, I really don't care what the rules are here, as long as I know where I can step and where I can't... that's all.