Different names
'What's in a name? that which we call a rose by any other name would smell as sweet.'
Marriage has religious/sexist connotations
There should be civil marriage for everyone regardless of sex, or there should be neither
Maybe it's just a difference of location. Here people can be married outside of a church with a justice of the peace, which many do, so no real religious connotation. Not sure what you mean by the sexist part. Seems civil partnerships could be considered quite the triumph for the gay community and to give to everyone would minimize it's poignance, or at the very least be rude, because like you say it only has a different name. If the name has no meaning, then it has no meaning.