In "UPDATE: Effects of Lifting Blood Donation Bans on Men who Have Sex with Men" authors Ayako Miyashita and Dr. Gary Gates estimate the number of men who would donate blood if the "gay blood ban" was relaxed at over 300,000 leading to an increase in the blood supply of 2-4% (600,000 pints) per year which could potentially save the lives of 1.8 million people.
The way the estimate is done is that Miyashita and Gates look at three different scenarios in which the current lifetime ban is changed in three ways: removed entirely, changed to a "deferral" lasting 12-months after sexual contact with another man and changed to a 5-year deferral.
In the US, 8.5% of men (10 million) say that they have had at least one male sexual partner since age 18. GSS data show that 4.1% of men (4.8 million) have had a male sex partner in the last five years, and 3.8% of men (4.5 million men) reported having a male sex partner in the last twelve months.
If the current MSM ban were completely lifted, we estimate that an additional 360,600 men would likely donate 615,300 additional pints of blood each year. If MSM who have not had sexual contact with another man in the past twelve months were permitted to donate, we estimate that 185,800 additional men are likely to donate 317,000 additional pints of blood each year. If MSM who have not had sexual contact with another man in the past five years were permitted to donate, we estimate that 172,000 additional men would make an additional 293,400 blood donations.To me it appears as if the report buries the lead! 8.5% of American men say they have had sex with another man since age 18 (i.e. as an adult). Does that mean that this percentage of the population is "gay"? Not really, since being gay is a complicated combination of identity, self-realization and resistance to social stigma. However, in my mind the significance of this figure is the demonstration that same-sex sexual attraction among men is more common than is usually reported to the general public.