BOSTON, Sept. 14 - In a sign that the legalization of same-sex marriage has changed the political landscape in Massachusetts, the legislature soundly defeated a proposed constitutional amendment on Wednesday to ban gay marriage and create civil unions, an amendment that lawmakers gave preliminary approval to in a raucous constitutional convention last year.

Wednesday's 157-to-39 vote by a joint session of the House and Senate partly reflected the fact that some legislators now consider same-sex marriage more politically acceptable, after a largely conflict-free year in which some 6,600 same-sex couples got married and lawmakers who supported it got re-elected.

The vote also reflected some lawmakers' reluctance to pass a bill that could either withdraw rights from already married couples or create a class of married gay men and lesbians and a class of those unable to marry.

Indeed, Senator Brian P. Lees, a Republican who is the minority leader and who co-sponsored the amendment, which received preliminary approval from the legislature in March 2004 in a 105-to-92 vote, said he voted against it Wednesday.

"Today, gay marriage is the law of the land," Mr. Lees said, noting that same-sex marriage became legal in May 2004. Voting for the amendment, he said, would mean "taking action against our friends and neighbors who today are currently enjoying the benefits of marriage."

Saying he had heard from over 7,000 constituents, most against the amendment, Mr. Lees added, "Gay marriage has begun and life has not changed for the citizens of the commonwealth, with the exception of those who can now marry who could not before."

Wednesday's vote also reflected a change in the strategy of opponents of same-sex marriage.

Last year some legislators who opposed both same-sex marriage and civil unions voted for the amendment because they considered it their best chance at preserving marriage for heterosexuals.

This year, after it appeared that the amendment would fail, many opponents of same-sex marriage started a citizens' petition for a stricter amendment that would ban same-sex marriage without creating civil unions.

The earliest that amendment, endorsed by Gov. Mitt Romney, could become law is 2008. Supporters must get 65,000 signatures, the votes of 50 lawmakers in two consecutive legislative sessions and the approval of voters in a referendum. Both sides expect a difficult fight.

Representative Philip Travis, a Democrat and opponent of same-sex marriage, argued Wednesday for the stricter amendment.

"The union of two women and two men can never consummate a marriage," Mr. Travis said. "It's physically impossible. We can't get around that."

In contrast to last year's long sessions packed with activists, Wednesday's session lasted two hours and drew smaller, calmer crowds.

Juan Carlos Huertas was one of a few dozen opponents of same-sex marriage singing Christian hymns at the statehouse. Mr. Huertas said that the idea that marriage is between a man and a woman is "written in the Scripture."

Nearby were about 200 supporters of same-sex marriage, some with T-shirts or buttons that said "I Do."

For weeks, same-sex couples and supporters met with legislators to present their case. Elaine Lamy, 49, and Chris Hannibal, 50, of Quincy, who married last year, met with Representative Bruce J. Ayers and Senator Michael W. Morrissey, who was also lobbied by the women's heterosexual neighbors. On Wednesday, the women saw the two legislators, both Democrats who had supported the amendment, vote against it.

Senator James E. Timilty, a Democrat who last year supported the amendment, also changed his mind.

"When I looked in the eyes of the children living with these couples," Mr. Timilty said, "I decided that I don't feel at this time that same-sex marriage has hurt the commonwealth in any way. In fact I would say that in my view it has had a good effect for the children in these families."