I had always noticed (as did our female co-workers) that my straight married co-worker had a great looking tight bubble butt in his perfectly fitting work uniform. He knew I was gay and so I jokingly mentioned that he had a nice ass. He just laughed it off. A few days later at lunch time I jokingly mentioned again about his nice ass, and he said to me, "If you buy me lunch, I'll show you my ass." I bought him lunch, then we went into the locker room. He put on a sexy show for me, pulling down his pants just past his ass to show me his underwear clad cheeks, then bending over to show off his ass straining against the material of the briefs, hands tugging at the work pants, slipping the pants up and down to tease me. Fuck, it made me so horny! Then came the highlight as he stood sideways, arched his back and slowly and seductively slid down the briefs for a full view of his round, tight ass, all the while making eye contact with me and making little grunting sounds. He pumped his hips in a fucking motion, clenching his ass with every thrust, and finally turned with his back to me and bent over slightly for a full view, with more thrusting and clenching. The sight of the arched back, his clenched ass and his hands tugging at the sides of the briefs was incredibly erotic. My own private male stripper show! He's such a great guy and a lot of fun to work with. I don't know what it is about the sight of clenched ass muscles, but it's my #1 fetish.