I am 19 a girl the age of 15 wants to have sex

The age of consent in New York is 17. If you have "sex" with her you could be charged with sex sexual misconduct which is a class A misdemeanor punishable by up to one year in jail, 6 years probation and/or a thousand dollar fine. You would also have pay a DNA sampling fee and a victim impact fee. Being 19 you would be treated as an adult and would not be treated as a youthful offender. Consequently, if you are convicted of sexual misconduct then you would be required to register as a sex offender. The foregoing could all happen regardless of whether she is a willing participant. This would be treated as a juvenile delinquency matter.

It would be illegal. People seem to focus on the 4 year rule while overlooking other statutes in the same Article of the Penal Law. You would be guilty of Sexual Misconduct.

It is illegal to have sex with anyone younger than 17 regardless of the age gap.

See https://en.wikipedia.org/wiki/Ages_of_consent_in_the_United_States#New_York. The technical term is 'jail bait."