EDIT!!! Found out some more Info about the Shirt and Slogan.
You may know Sir Ian as a fabulous Shakespearean actor, or as Magneto in the X-Men films, or Gandalf in Lord of the Rings, but he's been out and proud for a long time. In my efforts to track down the source of the image, I discovered first that it's part of a British education campaign from Stonewall.org, a British advocacy and lobbying group that McKellen helped co-found. The picture of McKellen shows him wearing a t-shirt with the slogan "Some People are Gay Get Over It" created by school children and used by Stonewall in the U.K. for school education and awareness about homophobic bullying, and the necessity of calling attention to bullying, and stopping it at schools. McKellen has tirelessly worked for QLTBAG organizations since his public affirmation of his orientation in 1983, on BBC's Radio 3. Shortly thereafter, McKellen and a group of activists founded Stonewall, using the name of the historic N.Y. bar as a beacon for their human rights advocacy and education campaigns.
The Some People are Gay Get Over It slogan was used on posters, educational materials, and postcards in Britain, featuring a variety of celebrities and endorsed by Prime Minister Gordon Brown, among others. You can still buy a t-shirt or order the posters and post cards.