While you may be joking, this isn't a particularly okay joke to make. Would you appreciate it if someone said "it's impossible for people of (your ethnicity) to be gay, because all the ones I know are straight!" (or vice versa)?
Asexuality is as much a sexual orientation as being heterosexual or homosexual, and should not be dismissed off hand because you have not met any or many asexuals yet.
in my experience, when one person in a community "comes out" as asexual, 100 other people "come out of the woodwork" expressing their asexuality, too. Some knew they were asexual and didn't talk about it, many others didn't know what asexuality was but now finally have a word for how they feel... because the first person was brave enough to talk about it.
And since this is the first in our community, perhaps we should give them as much respect as possible?