Later, I asked him, "So, you didn't seem surprised or weirded out. Were you already familiar with cups, or just inclined to roll with it?"
He thought for a second and said, "Well, I figured it'd break the mood to stop fooling around and say, 'It's like you're birthing a robot!' so I didn't say anything..."
I mentioned it to a few friends who use cups, and 'birthing a robot' is our favorite new menstrual euphemism.