Marriage researcher Kevin Andrews raises several concerns about changing marriage.
If governments remove the gender-integration requirement, they have no reason “other than superstition” to retain the number limit, meaning polygamy is fair game.
Kevin Andrews has warned that legalising same-sex marriage could lead to an acceptance of group marriage.
In his book Maybe 'I do': Modern marriage and the pursuit of happiness, out today, Mr Andrews also says that the assertion that redefining marriage will not affect other marriages is misplaced.
He writes that if same-sex unions are recognised by civil law, ''other arrangements can also be recognised. Once the state can no longer insist that marriage involves a commitment to a member of the opposite sex, there is no ground (other than superstition) for insisting that marriage be limited to one person rather than several.''
Andrews warns that if the government legalizes anti-gender marriage, it will set a precedent for intruding in other areas of family life as well.
''The state will be called upon also to create the social conditions to protect such unions. If the state can define marriage as something new and novel, it can define other arrangements. It can 'educate' people to accept this new arrangement, as has occurred in a series of cases in Canada.
''Freedoms, including religious freedom, subsequently come under attack. Even the understandings of 'mother' and 'father' are replaced by 'parent 1' and 'parent 2' or some similar language …
''Redefining marriage 'politicises' the institution in a dangerous manner,'' he writes.
Mr Andrews, with a long involvement in marriage education, has taken two years to write the book and has been researching the issues for 20 years.