Having Marc Jacobs’ ex-flame Austin Armacost pose naked in ads for People for the Ethical Treatment of Animals might be one way to persuade the Louis Vuitton designer and others to stop using fur in their designs. Armacost on Friday shot an ad campaign for PETA in TriBeCa, appearing on a runway in the buff carrying a sign that said, “Turn Your Back on Fur.” Armacost, who also starred in the LOGO reality show “The A-List,” dated Jacobs when he was an 18-year-old model after meeting him on MySpace. “Being a part of Marc’s world for a little bit, and seeing him use fur and hides from alligators for leather, then meeting Dan six years later, it’s all come full circle,” he told us while getting makeup applied to his body. PETA spokesman Dan Mathews said the gay community at large has avoided wearing fur, but “there’s gay stylists and designers that keep it visible.” The ad will appear in fashion magazines and circulars during Fashion Week in September and will also expand to London, Paris and Milan.