Demoted former cabinet minister Eric Abetz has used the offensive word "negro" while defending his opposition to same-sex marriage during a radio interview.

Speaking to 2UE's Justin Smith on Thursday afternoon, Mr Abetz – a strident opponent of gay marriage - dropped the dated slur, considered offensive to black people, when rejecting the suggestion that refusing to bake a cake for a same-sex wedding was akin to discriminating against Asian people.

"Those sort of analogies are quite offensive and that sort of analogy was completely debunked by Justice Clarence Thomas, the negro American on the Supreme Court of the United States dealing with this issue … who dissented on the issue of marriage as well," Senator Abetz said.

Smith interjected, questioning his use of the word, however Mr Abetz continued, saying "trying to bring race into it has now been completely dropped".