Peers backed a government bill paving the way for gay couples to marry. It is set to become law by the end of the week, with the first weddings in 2014.
Labour's Lord Alli said its passage meant "my life and many others will be better today than it was yesterday".
But Tory peer Lord Framlingham said the "ill-thought through" change had been "bulldozed" through Parliament.
Peers approved the principle of same-sex marriage last month, despite efforts by opponents to "wreck" the legislation.
MPs had earlier done the same, in the face of opposition from many Conservatives, the Church of England and other faith groups.
Supporters of the Marriage (Same Sex Couples) Bill wore pink carnations during Monday's proceedings in the Lords, during which a series of minor amendments - including one relating to pension benefits for same-sex couples - were proposed.
Government minister Lady Stowell said the bill "puts right something which is wrong" and had been improved by detailed scrutiny in the Upper House. "I can't claim to be a gay rights campaigner, but I am a firm believer in justice and fairness," she said.
But the Gay Marriage No Thanks group claimed it had been prevented from mounting an advertising campaign around Parliament after its truck was vandalised and its driver threatened.
After clearing the Lords, the bill will return to the Commons for a short debate on government amendments before the Commons begins its summer recess on Thursday. The bill must then receive Royal Assent before it becomes law.

The trio of Jack Carlin, Ryan Owens and Joe Truman repeated last weekend's win in Glasgow, again beating France into second place in Apeldoorn.
Emily Kay, who won the new-look omnium last week, finished second behind Kirsten Wild in the four-race event.
Chris Latham was second in the scratch race; Rachel James and Katy Marchant were fifth in the women's team sprint.
Britain did not qualify for the medal rides in the men's team pursuit, with senior academy riders Matt Wall, Ethan Hayter, Matt Bostock and Joe Holt recording the fifth fastest time.
Racing continues on Saturday with the men's sprint, women's keirin, women's 500m time trial and men's points race.
Find out how to get into cycling with our special guide.
Same-sex marriage in England and Wales is a step closer to becoming law after the House of Lords approved the change.
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Britain's men won team sprint gold on the opening day of the season's second Track World Cup in the Netherlands.