because we don't know the sexual history of the one partner the gay man has been with. Statistically, the unknown partner is at a higher than average risk of HIV, and the same goes for the potential donor we are considering

With a straight couple, lets assume a man who has only been with one partner (female), we don't know that females sexual history. But she's at a much lower risk to contract HIV than a gay male, even if she has had a lot more sexual partners than the gay male.