He claims the BBC was simply enforcing its standing policy, which avoids discussing the private lives of public figures when doing so is not "serving the greater good". This requires the BBC to analyse a situation and decide for the public whether or not it is in the public's interest to know about it. In a live discussion, this decision rests with the BBC's representative on the spot, the interviewer.
In this case it was right to allow Peter Mandelson's sexuality to be discussed, given the context. When the issues surrounding Ron Davies' resignation are clouded by homophobia and the inability of the Cabinet of which Mr Mandelson is a senior member to take a clear position, his sexuality and the Government's attitude to it is very relevant.
The BBC gag goes much further, however: it prevents mention of any aspect of Mr Mandelson's private life, regardless of the context: that is to say, before anyone can know whether or not it is in the public interest to mention it in a particular situation.
I understand the BBC's mandate is to "inform, educate, and entertain". It is not to censor.