ohIdoliketobebesidethe, and your point is?
Suppose a 19 year old flirts with a barmaid and she sleeps with him. He has every reason to think she is old enough (doesn't know she is working behind the bar illegally) but then finds out she was really 15. This is rape?
They can make all sex for money illegal if they want. Doing it this way is just stupid and unjust.
As for 'educated guess' it seems they are including kerb crawlers in this. You might well assume a woman standing on the street and not in chains was there of her own free will.
You would have to pay ME to go with a prostitute and it would have to be an awful lot of money too, but messed up laws only encourage contempt for all laws