The Findlaw website has this to say about civil partnership in the UK:

"There is, essentially, very little difference legally between a marriage and a civil partnership. The difference exists principally due to protests from religious groups about recognising same-sex couples and heterosexual couples in the same way. Therefore, the main difference between a marriage and a civil partnership is religion."

It is therefore essentially a discussion over terminology rather than substance, and the right for some members of society to call their relationship by the same name as the rest of us do. It is the religions which seem to put particular store on the term "marriage" without being very clear exactly what they see as being the essential defining characteristic which makes it special. Apart of course from the obvious point about gender, but this is only relevant if procreation is essential to the concept of marriage, which as we've already discussed is not the case.