Has anyone actually married their "backup" plan??

A lot of people seem to have these deals with their closest opposite sex friend (or same sex, if you swing that way, although personally none of my homosexual friends have such deals) that by a certain age if they're both single they'll get married. I'm just curious if anyone actually gets to that point and follows through with it. And if so what was the age, circumstances, why did you decide to really do it, etc.