I think the main reason for marriage is that it is largely based upon tradition, which relates to what you said of society expecting one to get married when the time comes. Another part is based on religion, many religious people think it a great sin to live with a member of the opposite sex, not to mention sex before marriage.
Personally, I kinda consider marriage to be an outdated practice but if I were seriously involved with a woman who wished to get married would be more that happy to oblige (plus my folks would be pissed if i didn't ).
Ever been licked by a pirate?