It's easier said than done. Think about your own secrets. Do you merrily tell people about it? I doubt that.
The beauty behind "best friends" is the fact that you can tell them right in their faces why you think it sucks. Find time, and talk things out in private. Tell him you're not comfortable him using you as an excuse of his sexuality. You may hurt his feelings but he'll surely bounce back
Also, I don't think your opinion of him letting the world know that he's gay matters; after all, why would he be using someone to hide it? Just put yourself in his situation. I know it sucks and hard to understand but for 1 sec put yourself in his situation. As they say, "you need to be a gypsy to understand a gypsy."