Of course gay and bi people should be able to donate blood, all people should be able to donate blood! It doesn't matter who you sleep with to determine if your blood is free of diseases so the blood can then be passed to another person who needs it. This can save lives and who you are attracted to doesn't change that.
I'm gonna go with yes, gay men should be allowed to donate blood; rather, the orientation of the donor should not be an issue. Blood donations should be quality-tested, not left to straight people. Anybody can get HIV, not just gays; they oughtn't be singled out and made ineligible for blood donations.