yeah marriage isn't the same as it was a 100 years ago, but what tradition is now a days?
either way, marriage still has it's place in today's society, it solves a crap load of paperwork if one of the two would die, and taxes for married people works differently.
Honestly, there is so much legal crap involved in marriage and often very different in different parts of the world that I wont even bother bringing everything up but point is there is often pretty good bonuses legally if you get married.
so today's society and system pretty much promotes you to marry a person you plan on spending a longer period of your time with, even if it isn't for your entire lifespan.
for example, believe me when I say... gay people wouldn't be fighting this hard just to be able to throw parties where they declare their love... might be one reason... but not the whole point.