The Illinois Religious Freedom and Marriage Fairness Act legally recognizes marriage between two persons, regardless of gender (750 IL Comp. Stat. Sec. 5/201 et seq.).
Reciprocity. Except for a common-law marriage, any marriage legally entered into in another jurisdiction is recognized as a marriage under state law.
Civil unions. Civil unions remain legal in Illinois. Any couple already in a civil union may apply for and receive a marriage license and have their marriage registered, provided the couple is otherwise eligible to marry under state law.
The U.S. Supreme Court subsequently issued a decision legalizing same-sex marriage in all states and requiring all states to recognize same-sex marriages legally performed in other jurisdictions (Obergefell v. Hodges, 135 S.Ct. 2584 (2015)).