In the broadest scope I've never understood why there has to be laws concerning marriage. It's a private contract.
Note that there are laws concerning private contracts in most countries.
I agree that marriage is just a contract between two people (note that I disagree with the contention that "love" has anything to do with marriage - no, you don't have to be "in love" to marry, nor does a marriage end just because you stop being "in love").
The devil, however, is in the details. What responsibilities does marriage imply (as a minimum, there are tax issues in most places), and what privileges does it grant (among others, there are medical issues most places - your spouse can make medical decisions on your behalf if you are incapable, for instance)? What are the limits on marriage (so, why limit it to two people, for instance)? And on and on.
Note that while some religions consider marriage a sacrament, there are very few that consider the religious ceremony legally binding in and of itself. And yes, that applies to most Christian sects (I used to belong to a moderately conservative branch of Christianity whose attitude toward marriage was "that's a State issue, but we'll perform the ritual for you AFTER you get the sanction of the State (marriage license))....