Is it legal for a 17 year old to date a 19 year old in California if they don't have sex?
Without sex or marriage, it's perfectly legal.
It is legal. As soon as she dings 18, it'll be legal for anything. In most places, however, you should be able to be in a relationship with an underage person so long as the years of difference don't exceed 5. So yes, it's legal for a 20 year old guy to date a 17 year old girl in California without having sex.