Human rights campaigner Peter Tatchell is backing calls for opposite-sex civil partnerships to become legal, as MPs debate a private member's bill on the matter.
Conservative MP Tim Loughton put forward the bill, as only same-sex couples are currently entitled to civil partnerships, which were created before LGBT couples were able to marry.
A petition has also been created in support of the move which has gathered more than 80,000 signatures.
Tatchell, who himself is gay, is supporting the bill, after previously backing calls for civil partnerships to be created in 2003.
He said: “It’s time for ‘straight’ equality. It cannot be right that same-sex couples now have two options, civil partnership and civil marriage, whereas opposite-sex partners have only one option, marriage."
Opposite-sex civil partnerships are already available in the Isle of Man and Tatchell asked "if the Isle of Man can have civil partnership equality, why not the UK?"
The campaigner said "in a democratic society, everyone should be equal before the law, with the same rights and responsibilities."
He described it as "outrageous" that for the last 14 years "governments have been unwilling to legislate equality."
Campaigners say this is particularly important in the UK as common law marriages are not recognised here. This means if one person in a couple dies, there is no legal right for the other person to attain their assets or wealth or receive tax relief.
Surviving partners are also denied guardianship of any surviving children, no matter how long the couple have been together.
The Equal Civil Partnerships campaign claims the state has a responsibility to protect children and couples who decide not to get married.