This is part of a series of stories examining the cultural, social and political currents swirling around marriage. In ancient Greece and Rome, marriage was primarily a way for the upper class to pass down family property. In early American Colonial times, a man and woman were considered married if they simply said they were. And it wasn't until about a century ago that the practice of marrying for romantic love became widespread. In the national debate over gay marriage, supporters say marriage is about love and commitment between two people, and that to deny gay men and lesbians the...