It is sexual assault because the 15 year old is not of the age where she can consent. However, there is a defense if the conduct was consensual and because of the short span between the ages. To prosecute, the gf must be cooperating or have at least given a statement saying that you had done the act; otherwise, they would have no evidence (unless the mom actually witnessed the act.)
Sexual penetration does not have to occur between a penis and vagina for it to be criminal. Any body part or object used to make contact with the sex organ of another person is a serious matter. If the police come to question you, you have rights. Use them. Respectfully refuse to answer any questions and tell them you want an attorney. It doesn't matter if the cops try to trick you by asking who your lawyer is and what his/her telephone number is...you don't have to even have a specific lawyer in mind. Just tell them you want one.
In Louisiana, the crime is "Indecent Behavior With Juveniles," La. R.S. 14:81.1.
The law requires an "age difference of greater than two years between the two persons." That can mean an age difference of as little as two years and one day.