I think you've got a lock on the answer already, Michah--it was all about context. Had you made your joke in more intimate company, it may have been better received, but to the sparkling hordes attending a baby shower, it fell like a brick. It's okay though, please believe me--I think we've all been in your position at one time or another. I can attest to several instances of my own making that I cannot forget to this day. A few have the power to crack me up in hindsight. The others may reach that point someday too.

Byzantine is right on, as usual--chalk it up to experience and soldier on.

All the best to you.