...is that I just got to have an amazingly awkward conversation with my mother (who is almost 70) about why people might find it pleasurable to be, in her words, "tied up and beaten".

I really wasn't sure what to say. Although we did cover that:

1. She does kind of understand why being tied up might okay; you are "sort of relinquishing control".
2. She understands that pain might release endorphins, which for some people might be enjoyable.
3. She thinks dominance and submission is weird, and tried to analogise it to a step father killing his step children (wut?)

Oh, and in an effort to try and explain that people like weird things that might be hard to understand, I mentioned the popularity of slash fiction among heterosexual teenage girls. Once I explained what it was (she'd never heard of it), she volunteered that "oh, that's not that weird". I must have looked surprised, because she reiterated: "no, I understand it". I didn't ask for more details.

So thanks E. L. James. I'll be sending you my therapy bills.