Japanese Prime Minister Fumio Kishida on Saturday called homophobic comments made by one of his secretaries "outrageous", indicating he could consider dismissing the aide.
Masayoshi Arai told reporters Friday evening that he "doesn't even want to look at" married same-sex couples, according to public broadcaster NHK.
Arai said he "wouldn't like it if they lived next door" and that people would "abandon the country if we allow same-sex marriage".
Kishida told reporters Saturday that Japan is "aiming for an inclusive society... that recognises diversity".
"The secretary's comments are completely incompatible with such policy and are outrageous," he said, adding that he would need to "take tough measures" against the aide.
The remarks came after the prime minister told parliament this week that same-sex marriage would "affect the society", therefore lawmakers need to be "extremely careful in considering the matter".
Japan is the only nation in the Group of Seven industrialised countries not to recognise same-sex unions. Its 1947 constitution stipulates that "marriage shall be based only on the mutual consent of both sexes" and "with the equal rights of husband and wife".
More than a dozen couples have filed lawsuits in district courts across Japan arguing that the ban on same-sex marriage violates the constitution.
In November, a Tokyo court said the country's failure to legally protect same-sex partners created an "unconstitutional situation" -- while ruling that the constitution's definition of marriage was legal.
Arai apologised late Friday, saying his remarks were not appropriate, even if they were his personal opinion.
If he steps down, it will be another blow to Kishida's government, which has faced plummeting approval ratings since last year.
Kishida has lost four ministers in just three months over allegations of financial irregularities or links to the controversial Unification Church.