I think that the culmination of a relationship in marriage varies in importance depending on one's faith, as well. Since I'm Catholic, I'd ideally marry whoever I'm in love with when the time comes, but I can understand why someone who doesn't identify as Christian doesn't feel compelled to "validate" their relationship with their significant other through marriage. If two people truly love each other, they don't need a priest or papers to tell them that they're in love. That being said, in today's society, it seems that marriage is seen as the culminating event in a romantic relationship and signifies the beginning of a new life as a family. However, I believe that with time, marriage will be seen less and less as the "familial gateway," so to speak, at least as far as the Western world is concerned.