Who the fuck thought this is a good idea?
Let me explain about “pre-existing relationship with minor”, bear with me.
It was supposed to be for “close innocent pre-existing relationships” that just happened to have an age gap, like childhood friends with 1-5 years age gap, close family acquaintance, distant relatives, or neighbors with close relationship between family, etc.
Basically a relationship between two people that the family already know and trust. If the two people are, let’s say have 5 years of age gap (which is completely normal). As soon as the older one reach adulthood (18) the relationship would suddenly be illegal, even though they grew up together, have healthy, innocent, and loving relationship, with a family approval. Without this law, the society would have to force them to break up. Which is unfair for both of them, they grew up together and have this wonderful relationship with each other but then suddenly when the older one reach 18 they need to break up because “the law said so”.
Basically this is the reason why that law exist. The reason behind that law itself is innocent, sweet even.