Terms such as mother, father, husband and wife could disappear from the law if gay marriage is legalised, MPs have been told.
Family First director Bob McCoskrie told the select committee considering Louisa Wall's gay marriage bill that Spanish law had recently replaced the terms mother and father with Progenitor A and Progenitor B.
"The US State of Washington is to remove the terms husband and wife from divorce courts," he said.
"In France the words mother and father will be stripped from official documents, McCoskrie tabled 24,000 extra signatures to petition against the bill on top of 48,000 already presented at parliament.
"I see marriage as similar to the Treaty of Waitangi. It is historical. It is the foundation of our culture," he said.
"It certainly shouldn't be altered."
New Zealand Aids Foundation told the committee that countries with less discrimination against gay people have lower rates of HIV.
"The internalisation of homophobia leads to a devaluation of self and a reduction in the ability of gay, lesbian and transgender people to make positive decisions," said doctor Jason Myers of the Foundation.