For you being at the age of seventeen years old (17) can consent to having sex. Sex with a person under the age of 17 cannot be consensual because the law presumes that someone under seventeen does not have the capacity to consent.
For example,
New York's sexual misconduct statute states: "A person is guilty of sexual misconduct when: 1. He or she engages in sexual intercourse with another person without such person's consent... ." If you have sex with a person who is 16 years old, and the person agrees to the sexual conduct, this is still sexual misconduct because the law presumes that the person cannot consent.
However, a 17 year old is not an adult and cannot go off and live on their own (unless the person gets a court ordered emancipation). Also, a person who is 16 or 17 can only get married in NY with parental consent. A 14 or 15 year old can only get married with parental consent AND court approval.
So the answer to your question: Is it illegal for you to date? No. This, I believe, this will evoke moral issues throughout the general community. I would strongly urge you to rethink your discission. Laws pertaining to children, minors, have increasingly become less tolerant in this area. The willingness to educate yourself on the facts is in itself very commendable. Obviously, you do have concerns with the circumstances of the relationship. Morally, I can only suggest that you put yourself in the place of her parental unit and ask yourself, "How would you feel in this situation?".