I guess the reality of the situation is that they'd rather have donors who have never had any form of sex...however since that would reduce donor numbers significantly, they have to allow people who have heterosexual sex, to boost the numbers, and yet feel that this gives them enough donors such that they dont need blood from the slightly riskier social group: homosexuals (That's not a homophobic statement, we all know that there is more HIV and such floating around the gay scene)