The pontiff said marriage is "a sacrament between a man and woman that can't be changed", but he supports civil laws that try to help those who have "a diverse sexual orientation".
Pope Francis has ended his trip to Slovakia by speaking out on abortion and same-sex marriage.
During his flight back to Rome he said "abortion is homicide".
But he backed away from denying communion to politicians who support abortion rights.
He also spoke about marriage and said it is a sacrament between a man and woman, but added that same-sex civil unions are good and helpful to many people.
The pontiff said the Catholic Church “doesn’t have the power to change a sacrament as it was instituted by the Lord”, but he supported civil laws that try to help “those who have a diverse sexual orientation”.
He said: "If a homosexual couple wants to lead a life together, the State has the possibility to give them safety, stability, inheritance; and not only to homosexuals but to all the people who want to live together (in a civil union)".
“This does not at all mean that they have to be condemned: No, they are our brothers and sisters. But marriage is a sacrament, between a man and a woman''.
Francis’s remarks came during the traditional inflight press conference returning from his September 12-15 visit to Budapest and Slovakia.