New Zealand Votes for Equality.

I seriously don't get what's SOOO important about marriage...
Other then religious stuff...

Marriage is not only religious, or else civil servants (judges, clerks; different offices in different countries) would not be perfoming or even noting it.

Marriage gives people certain automatic rights and obligations towards each other and their children.

For example, as Rick mentioned above: the right to adopt children together. If same-sex couples can't marry, that means any children they have are considered, by the state, to have only one parent.
Consider: a lesbian woman gives birth to a baby. Her partner is not allowed in the delivery room because she is not "next of kin". The two of them raise the child together, but the non-birth mother is not allowed to register the child for school, or get him a passport, or consent to medical procedures for him, because she is not his mother. If the mothers separate, the non-birth mother doesn't have to pay child-support, and has no right to visitation. If the birth mother dies, the non-birth mother doesn't automatically get custody. All this, while in the child's mind both women are equally his mothers.

And if you have a bio-bias and feel the birth mother is "more" mother, consider adoption: Two men adopt a baby. It isn't biologically related to either of them. But, in countries where men can't marry, only one of them can appear on the adoption papers. Only that one, picked at random or due to being a better match for some adoption agency criterion, is the baby's father. The other is nothing. Does that make sense?

Even without children, consider: If you were lying unconscious in the hospital, and someone needed to give "informed consent" to treatments for you: Who would you rather the hospital consider your "next of kin", your same-sex partner of 20 years, or your parents who disowned you 20 years ago when you came out to them?

All of these things, and many others (taxes, inheritance, insurance, etc), have nothing to do with religion. It's all about a legal status, paperwork, official records. And for these things, the genders of the two people getting married are irrelevant.