Emanuel will speak tomorrow at the U.S. Conference of Mayors meeting in Washington DC.
During that gathering, the group Mayors for Freedom to Marry will be launched.
On the group's website, more than 75 mayors have pledged to support same-sex marriage, including Emanuel, who says same-sex couples should share in the benefits of marriage and that there are too many benefits of marriage that are denied same-sex couples.
He says marriage is a recognition of love and the state should not be in the business of discrimination.