Cuckolding is a sexual practice in which someone is aroused by the idea of watching or listening while their partner has sex with another person—and it’s a surprisingly popular sexual interest. In the survey of 4,175 Americans’ sexual fantasies I conducted for my book Tell Me What You Want, I found that 52% of heterosexual men and 66% of gay and bisexual men had fantasized about this before. In addition, 26% of heterosexual women and 42% of lesbian and bisexual women reported having had this fantasy at least once. So why is cuckolding so popular?