The movement to legalise same-sex marriage is not about obtaining benefits and rights for same-sex couples, but obtaining public approval for different forms of sexual conduct and relationships, a leading US legal academic has said.

Speaking on the Pat Kenny radio show, Professor Robert George, Professor of Jurisprudence at Princeton, said that, if the movement were aimed at securing rights such as visitation rights in hospital, these could be secured “either by civil unions or civil partnership, or even without such a scheme, simply by permitting contractual relations, which are permitted under US law”.

Professor George, a former appointee of President Clinton to the United States Commission on Civil Rights, said that the battle “is more a battle about the nature of morality and the nature of human relations”.

He added that he would prefer to “debate it straight on the merits rather than debate it through what seems to me a false lens about benefits, which are obtainable in other ways”.

He mentioned the fact that there were now, according to Time magazine, 500,000 polyamorous groups in the US.

These groups had existed throughout history, Professor George said, “but they have been largely underground because they’ve been morally disapproved of”.

“Now we have a movement in the US to bring them above ground and to acknowledge them and even recognise them,” he added.

Professor George went on to say that “where marriage understood as the union of a man and woman with responsibilities to each other and the children that come from that union, where that has been honoured, we’ve been better off”.

Where this model has been “allowed to fall into decay, children have suffered,” he added.

Other forms of relationship, such as polygamy, where men had more than one wife, or the much rarer polyandry, where a wife has more than one husband, had proven themselves historically to be bad for women, Professor George said.

Professor George also said that the fundamental divide on this issue was not about homosexuality and heterosexuality, but about “permissive and very liberal views and sexuality and more conservative and traditional views”.

He also dismissed the suggestion that the fight against same-sex marriage was unwinnable.

Professor George pointed out that, in the 31 states where same-sex marriage has been on the ballot, it has been defeated.