If you are found guilty--even of a lesser sexual offense--you will not only serve time, but will be listed on the Illinois sex offender register. Even after you finish your sentence you may have difficulty finding employment and a place to live.It is not a defense that the two involved are in a relationship or plan to get married.
Criminal sexual abuse is a Class A misdemeanor offense for which the sentence can be up to one year in jail and a fine of $2,500. The sentence for aggravated criminal sexual abuse is 3-7 years in the Illinois Department of Corrections and a possible $25,000 fine.
Your situation is too involved for an online answer.
Using the written documentation you have is a good place to begin. Contact a local lawyer - many may give you a short free consultation - to discuss your specifics. Far too many variables exist in the short post you wrote for any further observation by me. Many attorneys have information posted here on Avvo.
You might find my Legal Guide helpful "How to Choose A Lawyer For You"
This depends on the specific crime that the person was charged with. If it was a class A misdemeanor, the prior answer is accurate. For a listing of all possible punishments in Illinois, go to: http://www.myillinoisdefenselawyer.com/illinois-criminal-code-and-laws/
You can figure out the type of crime charged by looking at the charging information, it will list the class of the offense, then go to the web-site here and look for the range of punishment associated with that class.
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[DISCLAIMER- This information is provided as general information only and should not be construed as specific legal advice as this website is not intended to provide anyone with specific legal advice. This attorney is only actively licensed in Illinois. In addition, the answer given above serves for educational purposes only and provides general information and a basic understanding of the applicable law. Anyone using the site expressly consents that there is no attorney client privilege between any person and any attorney responding. No attorney-client relationship is created with the furnishing of this information. This answer should not be used as a substitute for fully discussing the factual nuances of your case with a licensed professional attorney in the state where you are located.]
You need to speak to an attorney in person so you can provide more details. A sex offense could be a Class A misdemeanor involving fines and local jail time or it could be a Class X felony requiring at least 6 years in prison. There are too many variables, so you should determine what this person is actually charged with and call an attorney.