Stonewall has joined with groups including Amnesty International and the National Union of Students (NUS), to call on Westminster to legalise same-sex marriage in Northern Ireland.
Northern Ireland is the only part of the UK which still defines marriage as between one man and one woman.
The Assembly has rejected motions to redefine marriage five times between 2012 and 2015.
The province has been without a devolved Government since January 2017.
Last year, Secretary of State for Northern Ireland Karen Bradley reiterated that marriage should remain “a devolved matter that should be addressed in the NI Assembly”.
The Christian Institute’s Northern Ireland Officer, Callum Webster, said: “Stonewall is trying to take advantage of the current political instability in Northern Ireland to force its politically correct agenda on the province.
“But MPs should not be abusing the absence of a devolved assembly to impose such controversial and divisive legislation.”
The organisations issued a statement praising the UK Government for promoting LGBT lifestyles and, despite opposition from NI’s elected representatives, urged politicians to change the law.
Of the 193 members of the United Nations, only around 13 per cent of countries currently allow same-sex marriage.