Is it morally wrong to date a 17 year old?

Is it wrong to date a minor? Yes. Having sex with someone who is under the age of consent is a crime. In California, it is statutory rape.

Is it weird if a 17 year-old dates a 15 year-old?

Yes, its perfectly legal for a male of 15, and a female of 17 to date. You both are still minors, but until said person becomes 18, it would be illegal for said people to involve in a sexual intercourse.

Is a 15 year old dating a 17 year old wrong?

As mentioned above, a minors consent to sexual contact is irrelevant in the eyes of the law in California (in many other states, the age of consent is younger). “Dating” can certainly imply sexual contact, which is illegal between a fifteen-year-old and an eighteen-year-old.