Do we care?
So, Robbie Williams is not gay. We know this because the Evening Standard (not online) made this today’s banner headline after he won his libel action against The People and Stars and Hot Stars, which alleged that he had had affairs with men.
Now, I don’t have a law degree, but my layperson’s understanding is pretty much as at Wikipedia - that “English law allows actions for libel to be brought in the High Court for any published statements which defame a named or identifiable individual or individuals in a manner which causes them loss in their trade or profession, or causes a reasonable person to think worse of him, her or them.”
Of course, there is precedent for this: in 1992, Jason Donovan did the same thing to The Face magazine and won.
Of course, things were different in 1992. But, in 2005, is it really the case that the erroneous suggestion that a pop star is gay will make reasonable people think the less of him or damage his career?

I think that the legal technicality was that the papers claimed that he had lied in a book published about him by saying that he only slept with women and the lie itself is the problem, not the subject of the lie. Additionally, it could be argued that describing him as gay exposed him to homophobic hatred - which would also be actionable under the libel laws.
Not really, because to be actionable a false statement must expose the plaintiff to “hatred, ridicule or contempt” in the eyes of reasonable people, or in the old formulation “right-thinking members of society”. The law adopts a historically dependent construction of what is “reasonable”, so when gay sex was illegal saying someone was gay would have been actionable per se, but now not.
As you say, all the plaintif has to do in such cases is to point to any occasion, preferably in the recent past, when he said he was entirely heterosexual. He can then contend that the defendant is effectively accusing him of lying, which obviously is always actionable.
“is it really the case that the erroneous suggestion that a pop star is gay will make reasonable people think the less of him or damage his career?”
No, but it is the case that a court case like this will get into the papers and get Robbie’s name noticed (he types as a Robbie song starts on the radio!)