So, I caught a female moth some hours before I caught the male. I put them both in the same chest, with lots of room for eggs. I checked on them just now, and found a male moth, no female moth, and no eggs.
It looks to me like the female died of old age, without noticing her youthful suitor. Is this expected behaviour, or is it a bug? (Frankly, I was mildly surprised she was still alive when I found the male - seemed like more than a game day - but she actually lasted a while before disappearing (dying).)