This isn't a right, at least in the US (understood that this case is in the EU). I was born in Germany in the 1980s, so apparently, I am at risk of giving people Mad Cow's Disease. Virtually everyone I work with has been outside of the country within the last year, and they can't donate blood either. These agencies can be discriminatory for all sorts of medical reasons, both good and bad ones. This is ultimately a question of risk: In the face of a blood shortage (ever-present issue), what is the appropriate risk tolerance for potential donors? Unprotected sex with a lot of partners, regardless of sexual orientation, seems like unnecessary high risk. Barring homosexual people who are either not sexually active or are monogamous sounds like low risk that is worth accepting.