WASHINGTON, Feb. 23 (Xinhua) -- U.S. state of Maryland's legislature on Thursday approved same sex marriage and the state's Democratic governor is expected to sign it into law.

The state Senate voted 25-22 to approve the law, less than a week after the House of Delegates passed the measure in a tight vote. The state's Governor Martin O'Malley is expected to sign the legislation. He is a supporter of the bill. It will make Maryland the eighth state to legalize gay marriage.

The measure's opponents have vowed to bring it to referendum in November. They will need to gather at least 55,726 valid signatures of Maryland voters to put it on the ballot.

So far, seven U.S. states, including Connecticut, Iowa, Massachusetts, New Hampshire, New York, Vermont and Washington and the District of Columbia have legalized same sex marriage, but the issue remains highly controversial in the United States.