What makes someone a sex offender?

Depends who you ask. If you are a victim, then it is a crime. If you are arrested for doing it, you claim it is an illness. If you are a doctor treating a sex offender then you might think it is a personality disorder. If you are a lawyer, then it is a legal definition. Sex offenders rarely think of themselves as offensive. It also varies in different cultures. Perhaps child abuse leads to becoming a sex offender.