I don't think your sexual orientation should pose a threat to donating blood. This would be profiling gays as ineligible to donate blood and would be unconstitutional and immoral. Blood gets tested for HIV/AIDS and other sexually transmitted disease and or diseases period. Just because someone is gay doesn't mean they shouldn't be able to donate blood.
Just because someone's gay doesn't mean that they're riddled with sexual diseases. Gays will take the same precautions as straights when it comes to sex. Also, discriminating against someone because of their sexuality is just wrong. If somebody wants to help others you can't just tell them no because you think that what they like makes them unable to tell the truth about the healthiness of their blood.