Of course, historically, "traditional" marriage (and I am talking about thousands of years) was just that: a contract. It was 50% about controlling property (land, cattle, wives, children, slaves) and 50% about making advantageous political alignments with other families and tribes. After all, why fight for the land over the hill when you can just marry into it? The Hapsburgs built an ever-expanding European empire that lasted over 600 years mostly by marrying into whatever kingdom that they wanted to control.
That's what I think of when I hear people talking about "re-defining marriage". Hey, straight people, don't blame us - we weren't the ones that re-defined marriage to make it about love and relationship. You opened that door. We just walked through it.