For instance, if someone is so blindingly drunk that they can't stand on their own, it becomes fairly obvious to onlookers. And even if they say yes, when they're that drunk, they're too drunk to give consent.
In that case, how do you measure how drunk someone is? Fact is, if a girl was drunk but consents - if that's rape.... then the jails would be full! It's OT but you can't expect someone to breathalyse all their prospective suitors before getting down - if the girl is getting that drunk she should be going home (or not getting that drunk) rather than saying yes and then accusing someone of rape.
Never test for an error condition you don't know how to handle. -- Steinbach