I understand what you're saying, but I also think you're cheapening sex by comparing it in such a manner, to peanuts of all things. How about 'marrying for sex is like buying an airline and only ever using one aeroplane'? That's more like it. There are many facets to marriage, but only one of them is sex. Sex isn't peanuts. Probably.
1) Airplanes are very costly (very, very, very)
2) They take a long time to build
3) The manufacturing process of an airplane involves millions of parts (about 3 million for an Airbus 320)
4) They consume a lot of fuel (tons of gallons)
In short, airplanes are cumbersome and are high-maintenance. So you can't compare sex to airplanes.
Oh wait, this is not the argument thread. Please carry on...