Marriage as a sacrament
This was a 12th-century invention which, (like the concordats begun at the same time), was meant to extend the power of the Church.
“Alarms about the imminent erosion of the ‘natural’ order of marriage were sounded over the demise of antimiscegenation laws, the expansion of the rights of married women, and the introduction of ‘no-fault’ divorce. Marriage has survived all of these transformations.” — Margaret Marshall, former chief justice of the Massachusetts Supreme Judicial Court
The Vatican wants to base marriage on unlimited reproduction and so has tried to block marriage for gays. When the Irish voted to allow gays to wed — a form of marriage that the Vatican cannot control — it called this "a disaster for humanity". To this, an Irish camapigner for gay rights replied, “The Roman Catholic church’s views on sexuality have been a disaster for humanity.”