A senior churchman has warned that civil partnerships for gay people will undermine the institution of marriage.
Adele Rees and Julie Thomas Anderson took part in a commitment ceremony in 2003
The Civil Partnership Act gives gay couples the same tax and inheritance rights as heterosexual married couples.
A Swansea couple will be one of the first in Wales to register for a new gay civil partnership when legislation comes into force on Monday.
But the Roman Catholic Archbishop of Cardiff has warned that the new law will have negative consequences.
The Most Rev Peter Smith said: "What the Government should do in terms of public policy is support marriage rather than undermine it," he said.
"To put beside marriage an alternative or what appears to be a perfectly approved legal alternative lifestyle I think does not help the institution of marriage at all."
Julie Thomas Anderson and Adele Rees are preparing to exchange pledges that will be as legally-binding as a marriage.
The Gorseinon couple took part in a commitment ceremony in July 2003, but the new legislation now makes the union as legally-binding as a heterosexual marriage. From 5 December same-sex couples can give notice of their intent to form a civil partnership.
Ceremonies will take place from 21 December. Swansea's registrar office has already received 10 bookings.
Ms Rees said: "We had the commitment ceremony but we wanted something that was legally-binding.
"It's been a long-time coming - there were always going to be objections. It will still not be religious, but I would not want a religious ceremony anyway - although I think some (same sex-couples) would like that."
Amongst the rights civil partners will enjoy are the right to inherit a partner's estate, pension rights and paternal responsibility for each other's children.
Gay rights charity Stonewall Cymru has particularly welcomed the legal protection the new law offers to gay people in the event of a partner's death.
Stonewall Cymru's director Dr Alison Parken said: "Up until now, some same sex couples, if a partner dies, they haven't had the right to claim the body and take care of funeral arrangements.
"They have not been able to pass over survivor benefits whether from the state pension or occupational pension so there's a poverty issue there for a surviving partner."
One Swansea law form believes it is amongst the first in Wales to offer pre-nuptial agreements for gay couples.
Sali Jackson-Thomas, head of the Family Legal Team at JCP Solicitors, said she had already been approached by couples interested.
"Should the relationship fail the courts will be given the power to dissolve civil partnerships on pretty much the same grounds as divorces are grated in heterosexual marriages," she said.
"We believe this will lead to a rise in requests for formal agreements - similar to pre-nuptial agreements - to be put in place pre-registration, specifying who will be entitled to what should the relationship break down."