TANAISTE Joan Burton said the No campaign's posters showing an "ideal family" are "insulting" to people raised by single or adoptive parents.
Speaking at the launch of the Labour Party's Marriage Equality Referendum campaign, Ms Burton, who was adopted, said the No side was "fixated on the notion of the ideal family".
She said the No camp cannot win the argument on the "core issue" so have instead sought "to muddy the waters by dragging in extraneous and irrelevant issues into this debate".
"Thousands of people in our country have been raised by single-parent families because of the death of a spouse, because of divorce or many other reasons. Many others, myself included, have been raised by adoptive parents," she said.
"It is insulting to all the Irish people who are part of such family arrangements that somehow they are second class because they do not fit easily into some supposedly perfect family fit."
"Ours is a complex, diverse society that contains many family types and many forms of parenting. It is offensive and preposterous to suggest that children should be exclusively reared in one form of household only."
She also criticised the No camp for suggesting surrogacy was an issue in the same sex marriage debate.
Communications Minister Alex White, Labour's referendum campaign director, said the constitutional change would have "no adverse impact on children's rights and protections".
Mr White said marriage is about allowing two people to love each other and be treated in the same way as their family and friends.
"I think people understand that this proposition is fair, and I believe they will support it. This change will make an important statement about who we are as a nation. Yet it is not particularly radical," he said.
Labour Party chairman Jack Wall said research showed same-sex couples raise children just as well as heterosexual couples or single parents.
"The No side argument on parenting is not just plain nonsense. It is a deliberate, cynical lie," he said. "The quality of parenting in our complicated world is determined by caring skills and capacity for love. It is nonsense to suggest that, all else being equal, children are better reared in one form of household over another."