I guess the line comes with mental instability, disability, immaturity and such, at which point it doesn't really count as consent. Naturally, anyone who wants to commit cannibalism/be cannibalised is mentally unstable. The problem there is people will try to push what constitutes mental instability. It'd go from murder/suicide to weird ass fetishes through to anything that isn't heterosexual sex in the missionary position with the lights off for the purpose of procreation.