Can I make an official administratorial ruling the URL method is OK?
But seriously though, this does also cointain the question that what can be considered fitting* content for a message board that's not about porn. If a user wishes to share a picture of his (or her) genitalia that's perfectly fine with me. But if someone starts a thread on "share your favourite bukkake pictures" with images sourced from internet porn sites, my impulse would be to delete it as this is not a site for sharing porn. I wouldn't (publicly) share a link to the blog where I post my gay porn comic either, though it could be argued that is more OK than images from a bukkake site as I have created the drawings myself and we do have a forum for sharing artistic endeavours.
For the record, I'm not talking about any official TW policy instituted by yours truly here. Adult material is a complex issue, especially considering the generally anarchic nature of TW, and there doesn't seem to be an easy, clear-cut ansver to the problem.
* = Not the best choice of a term, but it's late and I'm tired, so bear with me.