"Mr Key should watch his language," he said.
"I'm currently touring secondary schools in UK, attacking homophobia in the playground and discouraging kids from the careless use of 'gay' which might make their gay friends (and teachers) feel less about themselves.
"So even as he supports the proposal to introduce same-gender marriages in New Zealand, I do hope John Key listens to his critics and appreciates their concern. Careless talk damages lives," he said.
Key yesterday said he meant "weird" when he used the word "gay" in the interview. He had picked up the term from his children and had no intention of sounding homophobic.
This morning he told Firstline he was just "mucking around" when he made the comments.
"There'll be times when it's a bit jocular and a bit of fun and you're mucking around on the radio. That was an example on Friday."
Key said he had always been professional when dealing with difficult issues such as the global financial crisis, but not every situation called for him to be serious.
"Where there have been very serious issues in New Zealand ... All of those I'd dare you to show me one example where I haven't discharged my responsibilities seriously, professionally and appropriately.
"But not every single situation is like that. Not every radio station you're going to go on is going to be like that. That is the nature of the role of being Prime Minister."
Though he agreed it was an unfortunate use of words, he said: "a hell of a lot of people would use it and the common use of it is in the Oxford dictionary".
Sir Ian said many words that can offend people are in the dictionary.
He said New Zealand should value its history of leading the world on many social and policy issues.
Key also courted controversy over reports he branded David Beckham "thick as batshit" while speaking to a group of students at Dunedin's St Hilda's Collegiate last Friday.
He has refused to confirm or deny insulting the football superstar.
However, British media have picked up the story.
Beckham's London-based media guru Simon Oliveira - who also oversees global campaigns for tennis ace Andy Murray and Formula One star Lewis Hamilton - was not impressed.
"It is not worthy of comment."
* Read McKellen's full blog post here.
- nzherald.co.nz with APNZ
