Trademark law is complicated. But the basic premise is to provide brand protection for companies. When a company trademarks a word or a mark, it has exclusive use of that word or mark to identify their product. Others cannot use the same word or mark to identify their own product, if their use would prove "confusingly similar" to the public.
Really, the very guts of trademark law is to protect the public from being misled as to the source of goods. If it says "Apple" on the box, and there's a computer in the box, but it isn't from that company in Cupertino, the public has been misled.
One test of "confusingly similar" is how similar the products are. But there are many other aspects to that test, and I gather it's the most ambiguous aspect of trademark law.
You cannot use a word or phrase that INCLUDES a trademark if it would prove confusingly similar. I would imagine that any term that includes "iphone" would be confusingly similar, so iphone5, iphone6, etc. would be off-limits, at least if it has something to do with mobile phones.
I think they would have more trouble proving that a porn site is trying to confuse the public into thinking that source of their porn product is Apple Computer.
That said, this isn't an action under trademark law. It's an action under the UDRP, which is a POLICY of ICANN that registries have agreed to conform to. While the UDRP does aim to conform with trademark laws around the world, it has additional provisions that go beyond trademark law. One particularly broad provision is the prohibition against registering a domain in "bad faith".
"bad faith" could mean somebody registering a company or brand name with intent to re-sell the domain to the company. It could also mean a competitor intending to prevent the company from registering a domain using their own trademarked term.
http://en.wikipedia.org/wiki/UDRP
BTW, iphone5, iphone6, etc. are already registered. Some are held through a privacy service, others are registered to large domain resellers that can probably absorb the potential legal costs. They will probably get some money if they are not greedy.
I have some experience with a "clean" transaction in this regard. Many years ago, I was the original registrant of the live.net domain. I used it for an outdoor webcam. A few years later I got an unsolicited offer $28,000 for the domain. I was a bit surprised at the price, but the transaction went through, and the new owner used it for about a year showing live webcams at ski resorts and travel information.
About a year later, I was checking on the cam (I was still feeding my cam onto one of the new owner's pages) and got redirected to a Microsoft site. I called the owner to report that his domain had been "hijacked", LOL.
I don't know how much Microsoft paid him, or if he knew something was in the works a year in advance...