Three US states narrowly passed referenda in favour of same-sex marriage on Tuesday, while a fourth saw a measure defining marriage as being between a man and a woman defeated by a slender margin.

The winning margin was an average of four points.

They are the first states ever to vote for same-sex marriage. Campaigners for man/woman marriage were outspent by eight to one thanks to huge donations to the same-sex marriage side by a series of wealthy donors.

The results mean that the pro-traditional marriage side has now won 32 out of 36 referenda instead of 32 out of the 32 to take place before last Tuesday.

Washington, Maine and Maryland all saw same-sex initiatives win at the ballot box, while Minnesota declined to pass an amendment which would have enshrined marriage as an purely heterosexual institute.

Washington, in particular, saw an influx of big donations from executives from leading Washington businesses.

Leading Northwest brands such as Costco, Microsoft, REI, Amazon.com, and many others signed a full page ad in this Sunday Seattle Times, endorsing same sex marriage. The Seattle Times corporation, a leading media group in the state, donated $90,000 in in-kind newspaper ads to support gay marriage.

A leading opponent to same-sex marriage discounted the victories as waged on uneven terrain. Maryland, Maine, Minnesota and Washington are four states which traditionally vote Democrat and where Democratic voters are more likely to back gay causes, said Brian Brown, president of the National Organization for Marriage (NOM).

Jeff Bezos, Amazon’s billionaire founder, wrote a $2.5 million cheque in support of Washington’s gay-marriage effort. Meanwhile Microsoft founder Bill Gates and chief executive Steve Ballmer wrote cheques for $600,000 each.  

The money spent this year was the most lopsided in favor of advocates of same-sex marriage thus far, according to the UK’s Independent newspaper.  

However, the pro-traditional marriage side outpolled Republican nominee Mitt Romney in all four states.  

In Maine, the pro same-sex marriage side won 53pc of the vote, while the traditional marriage side won 47pc, compared to 41pc for Romney.

In Maryland, the No side won 48.1pc of the vote, more than 12pc higher than Romney’s 35.5pc total, while in Minnesota, nearly 48pc backed the pro-marriage amendment, compared to Romney’s total of 45pc.

In Washington, 48pc opposed the same-sex marriage proposal, while Romney garnered just under 43pc of the statewide vote.

Overall, in the four states concerned, same-sex marriage supporters won 52pc of the vote, compared to President Obama’s 59pc, while supporters of traditional marriage won 48pc while Romney won 40pc.