So: Is it rape when you have sex with someone who didn’t tell you it was O.K., but told you it was O.K. earlier that night? Can you ever presume consent — just because you had sex with someone before, are married to someone, or even paid for sex?

In the view of Monica Burman, a Swedish rape expert at Umea University, “there is a sense of entitlement in certain contexts.”

“But there can be no mutuality,” she said, “about any form of sex if both parties do not consent.”

One reason this issue is so tricky is that it goes to the heart of how we define heterosexuality.

“There are still legacies of this idea that the male is the active pursuer and the female is the one who sets the limits,” said Ms. Kelly of London Metropolitan University.

In this world, as long as the woman is passive, she is consenting.