FEDERAL opposition spokesman on families 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.''

Mr Andrews' comments come after Parliament rejected two bills to legalise gay marriage and the issue caused trouble for Tony Abbott, who forced parliamentary secretary Cory Bernardi to resign over comments he made in the Senate suggesting gay marriage could be a slippery slope to accepting bestiality. Senator Bernardi also said the next step from having gay marriage was three or four people being able to enter a permanent union.