Actually, this works really well. My buddy (who is gay) hosted gay parties all the time, and his female friends threw themselves at him. "Come on, just do me a favor for tonight," was a line I heard the girls say to him every time. The trick is to convey it in relation to her: "You're gorgeous, but since I'm gay ..." or "Excuse me, I want you to know I'm not hitting on you, since I'm gay, but you are just so beautiful I wish I were into girls." or if you're feeling lucky: "You could turn me."