What is the difference between a civil partnership and marrige?

[lilibet]

If David and Julia get together in front of a registrar, say some nice words and sign some forms they are married.

If David and Julian do the same, it's a civil ceremony. Why?

Is the latest furore purely over the religious aspect of it or are there other changes?

[MammaBrussels]

This link from the BBC explains it quite well I think.