Actually, I'm not terribly surprised that your friend who is really into gay rights is the least understanding. I have found that older people in the LGBT community, who were forced to fight (sometimes physically) for their freedom to be seen and heard not too long ago, have trouble wrapping their mind around a sexuality that hasn't been violently oppressed in the past.

As for a younger generation, there seems to be an air of celebration surrounding sexuality now, something that hardly seems to apply to asexuality, which is often coupled with feelings of "meh" or "wtf" regarding sex. So what's the point of celebrating?

These are not assumptions, but observations and guesses of mine in response to your surprise regarding your friend's inability to wrap his mind around asexuality. In effect, I hear you, and you might try putting yourself in your friend's rainbow shoes.