"What word do we use to define that relationship between of two persons of the opposite sex .... that is ordered to providing a stable environment for the procreation of children and upbringing of children?"
In fact there is no word.
What the Cameron government is actually doing is abolishing marriage!
..... the intended change in the definition of marriage would mean that marriage as traditionally defined no longer exists. Thus heterosexual people would no longer have the right to enter into an institution understood to be only possible for heterosexuals, as doubly recognising both the unique social significance of male/female relationship and the importance of the conjugal act which leads naturally to the procreation of children who are then reared by their biological parents.
In effect, if marriage is now understood as a lifelong sexual contract between any two adult human persons with no specification of gender, then the allowance of gay marriage renders all marriages "gay marriages." ....