There are several reasons for a prohibited marriage in Illinois. The most commonly known is the age of consent law where someone is considered too young to be married, and the marriage is invalidated. However, there are other reasons that can prohibit you from getting married.
First, polygamy is illegal. People may not be married more than once at the same time in Illinois. So even if you are separating and going through a divorce, you cannot legally marry until your previous marriage has officially and legally ended.
Second, incest is illegal. You can't marry your child or parent, and you cannot marry your brother or sister. This is true regardless of whether your relationship is by whole-blood, half-blood, or through adoption. Though it's conceivable that if an adopted child or sibling could end that adoption relationship, they could marry a parent or sibling once the familial relationship is over.
Also, marriage to an uncle, aunt, niece, and nephew are prohibited. This only applies to blood relationships and not adopted bonds. However, cousins of the first degree can sometimes get married. Here is a little known fact: This is the case if the first cousins are 50 years of age or older or one of the cousins is sterile.
Third, same-sex marriage is illegal. People of the same sex cannot get married in the state of Illinois, though you probably know that they can legally join a civil union.
There are many reasons for prohibited marriage in Illinois. These reasons include age, polygamy, incest, and same-sex marriage. To learn more about marriage and getting a valid marriage license you may want to contact a family law attorney.