In high school and through her early 20s, Lisa Mattson dated a lot.

Inevitably, after a few dates with a new guy, the topic of sex would come up. If her date asked how many men she’d slept with, Ms. Mattson says she would “act demure and avoid the answer at all costs.” Sometimes she took a sip of her beer to stall. If all else failed, she’d say she didn’t want to talk about it.

“I’d...