The Gemara discusses which cases a person does't need hasraa (warning) to be chayav for and which do need hasraa. Why don't knas cases such as ones (rape) and mefate (seduction) need hasraa?
Hasra'ah is necessary only for physical punishment. Monetary punishments, including fines, never need Hasra'ah. This is because they are administered whether or not the perpetrator did his act knowing that he was transgressing an Aveirah. The money is meant as compensation to one that has suffered, rather than punishment for a transgression.