Jen since Sin wrote it - have wondered.
I wonder too! I know it's weird even contemplating anything to do with your parents and sex (or kink!), but it makes the most sense to me. They're probably not that much older than me tbh... They've lived a whole life that you would be completely oblivious about.
I think they threatened you with it as a kind of joke and now you're running with it and they're thinking 'Hell yeah! Best daughter ever!!".
You should confront your mum as an adult, pretend you know and just come straight out with it... "So did you wear it much? How often would you wear it?". Things like that. It might lead to a more meaningful conversation.