Do not talk to the officer anymore. Getting an attorney isn't a bad idea. If you are charged it is likely to be having a sex with a minor over the age of 16 which is a misdemeanor unless she is claiming it is forcible then it is a felony.
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Do not speak with anyone on the planet about this matter, except your lawyer. Hire one immediately. Deactivate all your Internet social media accounts today. Stop posting questions about the case on these legal forums. Sometimes the police will have the alleged victim call you on the phone and attempt to get you to incriminate yourself. This conversation is being recorded. If the alleged victim calls on the phone, hang up and call your lawyer. You have the right to remain silent and the right to counsel . . . use them!
You would be wise to hire an attorney to handle the discussions with the DA and any police investigator. You should not try to "explain" things yourself. You risk exposing yourself to criminal charges you don't know or understand. In Milwaukee you can often have lawyers work these things out in charging conferences. I would recommend you contact a lawyer immediately to be your mouthpiece. Don't go it alone on something this serious.
The other three answers are great. To answer your question about the validity of text messages, text messages can be used as evidence. The weight and probative value of this evidence can be challenged in a trial since usually nobody can confirm that you were the person with your phone at the time the message was sent and nobody watched you type in the message. However, cell phone records may indicate other texts and/or calls made or received during that same time period which may tend to show who was using your phone at the time. You should get a lawyer as soon as you are able to do so. Do not talk to anyone but a lawyer about your situation. You and your lawyer should talk about whether a meeting with law enforcement might be advisable to head off a bad situation before it gets worse. Being charged with a child sexual assault has several negative consequences even if you are eventually exonerated.
This communication is for the purposes of general advice only. This communication does not form any contractual obligation on behalf of the Attorney Stephen W. Sawyer or the Law Offices of Stephen W. Sawyer.