It's not just about statistics. For blood donations, if I had unprotected sex with a different HIV infected partner every day for a year, then stopped doing this for a year and one day, I would be eligible to donate blood.
If, however, I had sex with a man once in the last 30 years I am not eligible.
Why is it more sensible to turn away gay people then it is to turn away people who have regularly exposed themselves to HIV?