This, of course, is nonsense.
The phrase "male population" is usually taken to mean all males, not just those over 18.
So, the 2.8% figure was calculated by dividing the number of self-identifying gay men by that male population. To get back to the number of gay men, you have to take that percentage of the same population. The percentage would have been much higher had the
original calculation been done with the restricted populaton you chose to use to get a smaller answer.