A small study investigating stoned sex versus drunk sex reported on by the Washington Post suggests that overall alcohol is more closely linked with unsafe sexual behavior than cannabis, and that there is a distinct link between booze and sexual assault that is lacking with cannabis consumption.

Nonetheless, according to THIS recent article in VICE, some people think imbibing any amount of mind-altering substance takes away your ability to consent.  Legal precedent and modern dating culture suggest otherwise. The truth is murky.