The US State Department said it will no longer issue visas for same-sex partners of diplomats who work for the UN, IMF and World Bank, unless proof of marriage is shown.
The majority of 193 UN member countries do not legalise same-sex marriage, meaning diplomats face a tough choice.
They can conduct a same-sex marriage abroad, but face being imprisoned in some countries, travel alone, or break up with their partner.
A European diplomat based in New York told SBS News on Tuesday: “It’s tough enough being a diplomat with a same-sex partner, there are relatively few countries who will give a visa to your spouse or let them work. It limits your career options.

Germany's Foreign Minister Heiko Maas addresses the General Debate of the 73rd session of the General Assembly of the United Nations in New York. Source: EPA
"So it’s especially difficult to understand why a country like United States would take a backwards step on this and make life even harder on same-sex couples for no apparent reason."
Standardised practices
The Trump administration policy applies from Tuesday and affects same-sex partners of diplomats and staffers working at the UN global headquarters in New York. The partner must show proof of marriage by December 31 or leave the country within 30 days.
Reports suggest there at least 10 UN staffers in the US who need to get married by the year's end to secure a visa extension for their partner.
The US mission to the UN said the new requirement standardises its international visa practices, following the 2015 Supreme Court decision legalising same-sex marriage. Since that ruling, the US extends diplomatic visas only to married spouses of diplomats.
Heterosexual couples must now be married to obtain the G-4 visa. Applying the same standard to same-sex couples will "help ensure and promote equal treatment", a US State Department spokesperson said.
Previously, under a 2009 policy implemented by then-secretary of state Hillary Clinton, same-sex partners were granted a spousal (G-4) visa.
'Insidious impact'
Human Rights Watch said it is concerned the policy will "have an insidious impact" on same-sex partners from countries that criminalise same-sex marriages.
Deputy UN director Ms Kumar said: "The US government should recognise, as it had for almost nine years until today, that requiring a marriage as proof of bona fide partnership is a bad and cruel policy".
"[It is] one that replicates the terrible discrimination many LGBT people face in their own countries, and should be immediately reversed."
Samantha Power, a former US ambassador to the UN, said she was also dismayed by the visa policy, calling it "needlessly cruel & bigoted" in a tweet last week.

Samantha Power. Source: AAP
Akshaya Kumar said many UN staffers come from countries where same-sex marriage is illegal, and in some, punished by canning and prison.
"Only 25 countries provide for marriage equality, although Austria, Taiwan and Chile are expected to revise their laws soon," she said.
Ms Kumar said there are documented cases of death threats being sent to same-sex partners and their families who decide to marry abroad when the act is illegal in their home country.
She said the public nature of registering the marriage to "the required proof of marriage" for the visa endangers the lives of staffers.
She suggested a better alternative would be to require evidence be provided privately demonstrating a domestic partnership.