Presbyterian AFFIRM released a statement opposing same sex marriage, saying that the concept was "spiritually offensive to many Christian people, who still constitute a very significant proportion of this country's population and its voters."
Its spokesman Dr Stuart Lange said Affirm was a "widely supported conservative network" within the Presbyterian Church, although the Church as a whole had no formal position on the bill.
The statement was issued after at least 12 Presbyterian ministers voiced support for the bill by co-signing a letter from 57 clergy of the Anglican, Presbyterian, Methodist Churches rejecting the Catholic opposition to the bill last week.
Dr Lange said he believed the contrary views expressed by other Presbyterians were "isolated voices" and the majority of active Presbyterians wanted marriage to remain between men and women.
The Affirm statement said that a families with both a mother and a father were still the best way to raise a family.
"In a country with so many social problems, Parliament must be careful not to allow a further undermining of marriage by confusing its meaning."
The bill is expected to have its first reading next Wednesday and is likely to pass to a select committee although the numbers are not yet guaranteed.
In a Herald poll of MPs, 56 have so far confirmed they will vote for the bill but 48 were undecided or refused to say. Of those in support, several restricted their support to the first reading only and will decide whether to support it further after the select committee.
Other MPs are gradually making up their minds as the vote looms.
Justice Minister Judith Collins confirmed she would support the bill this week.
MPs have been issued with proxy vote forms to fill out in case they will not be in Parliament to vote at the time.
The Prime Minister - who has said he will support the bill - will be in Rarotonga but a spokeswoman said he had already filled out his proxy and handed it in to the party whips.
The MP leading the bill, Louisa Wall, said the bill's success could depend on NZ First. If it abstains, only 57 votes will be required for the bill to pass. However, 61 will be needed if NZ First votes against the bill.
Mr Peters again would not clarify whether the eight MPs would abstain from voting or vote against the bill, saying only that the party believed it should be put to a referendum.
"We are saying there is no reason why Parliamentarians should usurp the public rights to have a say on public morality issues. If we look at the circus of the drinking age, the problems Papatoetoe is having with prostitution and the alcohol laws of this country, it is clear that the public has been totally sidestepped."
It is understood they are likely to vote against the bill because of the risk of a backlash from NZ First supporters and members if the MPs give up the chance to defeat the bill by abstaining.