SPRINGFIELD — The state’s top court has upheld a decades-old precedent denying certain legal rights to unmarried couples.In a 30-page decision this morning, the Illinois Supreme Court ruled that a policy against treating unmarried cohabitants of a home the same as married couples remains “good law,” and that “the current legislative and judicial trend is to uphold the institution of marriage.”Justice Lloyd A. Karmeier authored the 5-2 majority opinion, which stemmed from a property fight …