(Re-mix of reply to acd)
I really don't think a lot of either religious, or secular baggage really helps when dealing with a highly distinct and necessary form of relationship: the child-rearing relationship.
Religion exalts marriage with its highest ideal, calling it holy; secularism exalts marriage with its highest ideals: social acceptance, romance, commitment, security, legality, status, recognition, etc. etc. These things are good, but they distract from the basis of the relationship they are trying to benefit: two people committed to each other in the service of conceiving and raising other citizens well. These couples are obliged, both morally and legally, to care for their offspring - so they are really sacrificing quite a lot for the good of society.
Getting way-laid (no pun) on some of the religious, or social trappings of marriage distract from the role it plays in transitioning a society from one generation to the next. Talking about sanctity, acceptance, romance, benefits, recognition, etc. can make it sound like marriage is about these things, so that weddings become more focused on religiosity, bridal extravagance, taxation coupes, etc.
So how do you deal with heterosexual couples commiting themselves to each other in civil unions, who end up having children?
They can can remain as such - with some particular type of benefit going to the principal guardian of the child - or, if they wish, they can then undertake marriage; that is, a commitment (freely undertaken) to cooperate in raising the child. Benefits should be forthcoming because marriage implies personal obligation and great personal cost in terms of time, money and concern. The difference between marriage and civil unions would be the level/extent of commitment intended in child-rearing.
Sterile married couples should still be recognised as such, for having offered their commitment; much like the war veteran who never saw action. Naturally, benefits ordered towards assisting in child rearing would not be forthcoming.
Adoptive parents are a distinct category, as both married and civilly united couples - or even individuals can adopt, with varying degrees of domestic commitment.
In any case, committed couples intending to bear/raise children needs to be treated as a distinct category. Perhaps if the term 'marriage' is further over-extended (it already is in respect of heterosexuals who do not intend to raise children), a new category must be envisioned by the law.
My suggestion would be that the category be somewhat disparaging in its terminology, to avoid the build-up of desirable social baggage which is fueling the current debate: perhaps 'committed breeders', or 'child bearers' - although disparaging terminology would seem unfit, because such people really do play a very important inter generational role in society.
