demerit points are lost when a driving offense is committed. ie. speeding, driving through a red light camera, drink driving etc.
But in a colloquial context, that of V plates, they refer to other sexual acts that have been committed (apart from the big one). comparable to 'the bases' or 'scoring'.
1:"Do you still have your V plates?"
2:"Yeah, but i've lost a couple of demerit points tho... ;) "