Plans to recognise marriage in the tax system were in the Conservative manifesto, however the Lib Dems have long opposed the idea and under the terms of the coalition agreement are permitted to abstain on the measure if it is ever brought to a vote.
"I have always strugged to explain to people why someone who is not married should pay more tax than someone who is not married," Clegg said on Tuesday.
"I think that it would be hard to explain to people why people who are not married should be stung with higher tax.”
It has been reported that the chancellor plans to introduce a tax break for married couples worth £150 - in part to appease Tory MPs unhappy at the coalition's gay marriage legislation.
Tory backbencher Andrea Leadsom asked during a Commons debate on gay marriage in December that culture secretary Maria Miller "articulate her support for marriage in Cabinet by supporting it in the tax system".
She told HuffPost today: "Marriage is special. Statistics as well as human instinct clearly show that the commitment of marriage provides the nurture needed to raise secure children as well as the emotional support needed by each partner.
"Cohabiting is not the same. The tax system should support what is 'good' in our society - tax breaks to encourage marriage are a great deal for the taxpayer and a great deal for our society."
In December Treasury minister David Gauke told the Commons that the government's commitment to introducing a proposal to recognise marriage through the tax and benefits system "remains firm".
"We want to show that we value commitment, so we will consider a range of options and advance proposals at the appropriate time," he told MPs.
Clegg's comments today will irritate many Conservatives who will worry the Lib Dems are set to fight any move to give married couples tax relief even though they are allowed to abstain on the vote.
Last month former Tory children's minister Tim Loughton told HuffPost he wanted to see George Osborne introduce a full marriage tax allowance as soon as possible.
However he said he feared “Lib Dem pet projects” meant that any tax cut for married couples would likely be a “half hearted, token measure that wouldn’t cut the mustard”.