Britain is moving towards equal marriage but gay and lesbian couples already have access to civil partnerships. So is it really the case that 'separate but equal' is not truly equal?
As Conservative equality minister Maria Miller introduces The Marriage (Same Sex Couples) Bill for England and Wales today (25 January), we examine the differences and see what impact the new bill will have, if it becomes law.
Though same-sex couples in the UK have had civil partnerships since 2004, marriage is currently restricted to opposite-sex partners.
Opponents of the bill have said civil partnerships are the same or similar to marriages and extending marriage to same-sex couples is unnecessary.
They argue partnerships have a 'separate but equal' status, giving same-sex couples the same rights as straight couples.
However there are key differences between the two kinds of union.
Civil partners do not have the same pension rights as married couples. If one civil partner dies, the pension share that the surviving partner receives is often lower and lasts for less time than with married couples.
The reason for this is the pension a surviving partner is entitled to is measured differently depending on whether they have been civil partnered or married.
For civil partners, public sector schemes are dated back to 1988. For private sector schemes, it need only be backdated to the Civil Partnership Act 2004.
But for married couples, a surviving partner is entitled to a pension based on the number of years their spouse paid into the pension fund.
This video, taken from Freedom Is Not The Problem, explains more via the medium of Lego.
Separate is not equal
The culture of many countries, including the UK, revolves around marriage as an institution.
Though not all straight people do get married, marriage with a loved partner is an option that is legally available to them.
Advocates for equal marriage speak of the emotional resonance marriage holds. Marriage represents a form of inclusion within society.
It is a way of belonging to a cultural group and buying into values seen as respectable and acceptable.
The language of marriage carries a social weight that civil partnership does not. This is a reason why civil partners often refer to their partnerships as 'marriages', rather than 'partnerships'.
Those who back equal marriage say using a different word indicates that society attaches less importance to civil partnerships.
Travel restrictions apply to civil partners but not married couples.
Countries like Sweden, Argentina and Portugal, where same-sex marriage is legal, do not see civil partnerships as marriage.
This means UK civil partners living abroad do not enjoy the same rights as same-sex married couples in the 11 countries where equal marriage is legal.
In addition, the marriages of foreign gay couples who travel to the UK are not legally viewed as marriages.
Gender is written into the structure of UK marriage law.
This means if a married trans person would like to get a Gender Recognition Certificate as part of living in their preferred gender, they must divorce and reapply for civil partnership.
Separate marriage and civil partnership regulations mean gender and orientation are the deciding factors in what relationships are legally recognized.
Speaking to Gay Star News, Liberal Democrat councillor and transgender rights campaigner Sarah Brown said: 'The lack of marriage equality historically has created a huge legislative mess for trans people.
'We have to end our existing relationships if we want official recognition of our gender and anyone who doesn't identify as either male or female has to lie when getting married.'
The current laws presume that gender is a binary and there are no people who identify outside 'man' or 'woman'.
On the new bill, Brown added: 'Those in a civil partnership will still have to end it, non binary people will still have to lie, those who had our marriages confiscated aren't getting them back.
'The bill is also introducing the ability for spouses to veto legal recognition of the trans partner while they're still married.'
Some say as much as 0.2% of the population is intersex and there is a growing community of non-binary trans and genderqueer people. For them a 'separate but equal' system does not necessarily reflect a more complex reality.
Official forms such as the UK census require a declaration of marital status.
This often means that civilly partnered people are forced to state their sexuality, ticking a separate box that says 'civil partnership' rather than 'marriage'.
Adultery and vows
Unlike marriages, consummation is not a legal requirement of civil partnerships. Neither is adultery recognized as grounds for dissolution. These differences won't be addressed by the new equal marriage legislation as the government does not intend to re-write these areas of law.
There is also no requirement for civil partners to take any vows.
These variations mean that UK marriage law doesn't necessarily cover civil partnership, opening the door for different treatment of gay couples.
Straights not included
Veteran gay rights campaigner Peter Tatchell has complained the UK government is not offering civil partnerships to heterosexuals too, creating another inequality in his opinion.
He said: 'Marriage equality bill is not true equality. It keeps ban on straight civil partnerships. Wrong!'
Tatchell advocates the extension of civil partnership to straight couples as well as marriage to gay couples.
Members of Parliament will vote on the new marriage bill for England and Wales for the first time on 5 February. It will then have to pass through various stages in the Commons and Lords before being sent to the Queen to sign into law. Separate legislation is being developed in Scotland.