But this is a law. This is not about whether the people involved can tell, this is about whether the judge or the police officer can tell in a situation where there are very likely no other witnesses and no recordings of the situation.

Also, if you think that the average couple who is enthusiastically in favour of having sex always does something like asking in explicit terms “Do you consent to do x, y and z?” before having sex (and, since consent can be withdrawn at any time, during sex) you are living in a very simplified world.

Of course this isn’t a problem for regular people because regular people are quite sensitive to their partner not being in the mood for sex or not interested in general in sex with them and will back off but these laws are not about regular people like you and me. These laws are about people who are going to, deliberately or not, ignore their partner’s feeling on the matter and for that kind of situation “I know it when I see it” just doesn’t really cut it.