What is marriage, exactly?

From "Japan braced for divorce epidemic as 'salarymen' hang up their suits" (The Independent, 4 May, 2006).

One of Japan's top-selling weekly magazines, Shukan Bunshun, recently peered inside hundreds of baby-boomer households and was shocked to find that many middle-aged women were practising their farewell speeches.

"To my husband: Don't suddenly get friendly with me after all these years of leaving me alone now that you have retired from your company. It's too late now!" said one 55-year-old woman who contributed to the magazine's survey.

I know marriage is a partnership of some kind, but a partnership to what end exactly? Should it have an end-date (I mean, a more flexible one than death)?

What exactly do we owe to our partners? Who decides the quantity of what is owed? How do we know we're giving our partners exactly what they're owed?

Are these even the right questions to be asking? Is marriage as we know it soon becoming obsolete, and therefore not worth pondering over?