Of course, you can have that without getting married. But you see, more than anything, marriage is really a cultural institution. It has a strong symbolic power. It's an excuse for a big party, a signaling of commitment, etc.
We humans do a lot of things based on tradition and culture. Marriage is one of those things.
Because come on, if we don't have culture, what do we have?
There is no such thing as logical action independent of values, which are cultural. Logic can help with determining how to serve values, but it can't tell you what your base values should be.
It only takes one exception to disprove a rule, and in the interest of not inviting debate - I'll pick an obvious example: biological imperatives have nothing to do with culture.
Everytime I hear somebody excuse silly behaviour with "culture" it reminds me of the Louis CK bit: http://youtu.be/CQSRPMFDTSs
We humans do a lot of things based on tradition and culture. Marriage is one of those things.
Because come on, if we don't have culture, what do we have?