There are several issues going on here, I have broken them down to deal with them one by one.
Links to content suited for more mature individuals
Definitely remove the link and replace it with some placeholder text.
The Stack Exchange Content Policy states:
Sexually Explicit Material. Accounts that use Stack Exchange to post sexually explicit or pornographic material, or links to it, will be suspended.
This has also been discussed on other sites, and as per Jeff Atwood's answer:
No, I don't think we want this, as it would cause us to get globally banned from web filtering software.
So including adult links, or even mentioning adult sites by name, is explicitly not allowed.
This particular question
I think it's of such a low quality anyway that without the link there isn't enough for it to remain open as per our rules on identification questions. Since the link is inappropriate it should be up to the user to provide further information to be used for identifying the movie in order for it to be reopened.
The "on topic" status of particular subjects/content
Regarding the particular subject, I personally wouldn't want to see those questions and I don't think it would narrow our scope much to disallow them but I can appreciate the grey area it would be leading us into.
If we go down the route of banning certain themes then it becomes difficult to draw where the line is, violence and sex are allowed but others aren't?
We already have a discussion on whether questions about the pornographic industry are on topic or not and the consensus at that time was that questions about the industry are on topic. Provided content stays away from being sexually explicit as per the content policy they should be allowed.
We have already determined that identification questions for pornographic material are off topic, but I'd like to add that in conjunction with the content policy, identification questions for sexually explicit materials are inherently off topic, since you can't provide enough detail to make the question on topic without breaking the content policy.