The Labour MP leading the charge on gay marriage says she believes "hang ups" over homosexuality, especially among older New Zealanders, are behind opposition to her marriage equality bill.

Louisa Wall gave the opening submission on her Marriage Amendment Bill to parliament's government administration select committee on Wednesday.

Ms Wall says despite homosexuality being decriminalised 26 years ago, a section of society "still has a residual issue about homosexuals and homosexuality".

"I think one of the motivating factors for the opponents against this bill is that fact that they still haven't acknowledged or appreciate that homosexual New Zealanders have the same rights and privileges as non-homosexual New Zealanders," she told the select committee.

Ms Wall said it was a "generational issue".

"For younger New Zealanders, the issue of marriage equality is one where they actually can't see what the problem is. So for them it is about principles of equality and non-discrimination. They haven't got the hang ups over Homosexual Law Reform that I believe some older members of New Zealand society have."

Ms Wall says marriage is a "right of citizenship", and her bill would remove the discrimination in marriage law.

"There are a number of people who contend that homosexual New Zealanders should be satisfied with civil unions, and what my bill essentially seeks is for them to have the choice."

She dismissed concerns her bill would "redefine marriage" or that it would force churches and marriage celebrants to take part in homosexual marriages.

The law would simply be a change at the state level, where the marriage licence is granted, she said.

Ms Wall also disputed the argument that allowing gay marriage is a "slippery slope" to polygamy, incest and bestiality, pointing out those are all covered by the Crimes Act.