WATCH: 'Traditional Marriage' Doesn't Mean What You Think It Means

Marriage has been thought a lot of changes over the years, and during that time it's never been as simple as "one man, one woman." Marriage is a constantly shifting, evolving institution, and over the last few thousand years it's seen radical changes to the roles of women, ages of the particiants, inclusion of different ethnicities, rules about violence and property, and even whether the spouses could expect to love each other.

But there's one quality that's always stayed the same: marriage has always been an agreement between people. So when societies change, and the people in those societies change, their marriages change along with them.

Usually that's a good thing. For example, as civil rights for women and people of color expanded, the rules of marriage became more egalitarian and fair. Now, the rules are changing again to accommodate same-sex couples. And just as conservatives predicted in the 1920s that marrying for love would destroy the institution, and racists of the 1960s argued that interracial marriages violated their religious freedom, anti-gay activists of today are using some very familiar arguments.

Watch below for a full rundown of the strange, sometimes shocking, sometimes wonderful history of marriage: