I would guess around 6-7%. If we assume that the distribution of sexual orientations is the same across race, nationality and culture (an assumption I'm sure Ahmahdinejad would argue), then the percentage should be the same as that in the US. A study conducted by the National Gay and Lesbian Task Force found that 5% of voters in the 1998 midterm elections self-identified as gay, lesbian or bisexual. Of course, the voting population doesn't match up exactly with the general population.
I would guess 6-7% of the world is openly gay, but I would say there are so many closet cases that I would guess 40-50% are at least somewhat bi or gay