While the authority said there were "sincere concerns of ... offence caused by the brochure", the allowance for "robust expression of belief or opinion, irrespective of the message", meant the pamphlet was allowed.
"Accordingly, the [authority] chairman said that there was no apparent breach of the Advertising Codes and therefore no grounds for the complaint to proceed," the authority said in its decision.
The authority's finding comes as opponents to the Marriage Equality Bill make a last-ditch bid for politicians to vote against the legislation.
Crowds are expected to fill Parliament's galleries for the historic moment on Wednesday when the bill is tipped to pass its final reading.
Family First director Bob McCoskrie said more than 15,000 people had signed a web-based pledge to vote against any electorate MP who supports the bill, and against any party whose leader supports it.
The Family First brochure, on the party's website, lists "21 great reasons" to not change the Marriage Act and asks readers to "help us find 21+ more politicians who will stand up for marriage".
One of the points on the list - titled "What next?" - says that if marriage is redefined once, it could be redefined again to allow polygamous and "incest-type" marriages.
Other points in the pamphlet include "equality is not sameness", "mum and dad matter", and "an ideology forced on all".
The authority said the code of ethics allowed for "robust expression of belief or opinion"; however, it had to be "clearly distinguishable from factual information" and an advertiser's public or political interests should be clear.
It said the "right of freedom of expression ... is not absolute as there could be an infringement of other people's rights".
"People have the right to express their views and this right should not be unduly or unreasonably restricted by rules."
Marriage Equality spokesman Conrad Reyners said although the pamphlet had caused offence, its arguments were weak and were not slowing the bill's progress through Parliament.
