Why can't men who have sex with men (MSM) donate blood?

This week, my school hosted a blood drive. Gay men were not allowed to donate. I was so repulsed by this that I, a female friend, refused to donate just on principle. Is there some reason why gay men are still excluded from blood drives? Are they still so much more likely to be HIV positive than heterosexuals? Is this a policy from a time before blood screening was required? Someone said it was some sort of economic benefit. It seems to me that the amount of blood they would get from including such a huge percentage of the population would make up for the maybe one percent (this is what I'm unsure of) that they had to take, screen, and eliminate. There's such a shortage of blood donated in the U.S. Why are we refusing so much healthy blood?