I didn't have a good sense of my sister's sexual experience. I knew she'd had boyfriends and I was sure they'd done more than kiss. I didn't bother asking because she was my big sister, and it was none of my business. She certainly never inquired about my (meager) dating history.

Now, though, intimately on top of each other, I wondered if this was something my sister had shared with someone else. We say 'virginity' like it's a singular thing and I suppose it can be. But there are all sorts of 'virginities' if you think about it -- an endless number of intimate acts we can experience for the...