The bill, a copy of which was obtained by The Washington Post, would amend the law to say "marriage is the legally recognized union of two people" and that "any person who otherwise meets the eligibility requirements ... may marry any other eligible person regardless of gender."
Bill sponsor David Catania, one of two openly gay members of the D.C. Council, said he had 10 co-sponsors, all but assuring passage, the Post reported Friday.
The bill also would have to survive a U.S. congressional review before it could become law.
Catania said his bill would not require religious organizations or their officials to perform a same-sex marriage or provide wedding-related services to same-sex couples.
"I think it is very important for people to realize we are talking about a civil marriage, not a religious marriage," Catania said.
If it becomes law, the bill would eliminate domestic partnerships, although couples already registered could either keep their partnership or convert it for free to a city-sanctioned marriage, beginning in 2011.
Tom McClusky, vice president for legislative strategy at the conservative Family Research Council, said "a number of legislators are looking at different things" in preparation for a congressional fight on the matter. Meanwhile, gay rights activist Peter Rosenstein said he and other advocates are banking on Democrats in Congress to deflect opposition.