The Trump administration on Monday began to implement its new policy [memo, PDF] of denying visas to unmarried, same-sex partners of foreign diplomats and employees of the UN.
The G-4 Visa [US Legal backgrounder] is used for family members of foreigners working in the US on behalf of a foreign government or international agencies. The visa also allows for family members of the foreigner to study while in the US.
The new policy provides:
Currently accredited same-sex domestic partners of United Nations officials who wish to maintain their G-4 visa must be ready to submit proof of marriage by 31 December 2018. After 31 December 2018, they will be expected to leave the United States within 30 days unless they submit the required proof of marriage or have obtained separate authorization to remain in the country through a change of non-immigrant status.
The Trump administration has stated [USA Today report] that the new policy is more in line with existing policy that does not allow unmarried opposite-sex partners to participate in the visa program. However, the shift is controversial as many countries do-not allow same-sex marriage.