Update to this: I got the following email in response to my complaint today. Not perfect, but it's a start, and I think it's a good opening.
Dear Kythryne,
We apologize. You are right and we should not have had the "ha,ha" on the comment. It was inappropriate and we are sorry that we have offended anyone.
This section is optional, so we do choose to leave it on the site as it helps us understand who is using the site. But it is optional for members to select - or not.
Thanks for pointing this out to us. BTW, we are not a large company. Just 3 of us trying to make a difference for book lovers.
Sincerely,
PaperBackSwap Team
Since they didn't seem to grasp the idea that there are NOT only two genders, I'm going to follow up with a bit of gentle education. (And check to see if they've removed the comment.) Still, pleased that it at least got a response and an apology.