In the cultural wars secular liberalism continues its slow, laborious march toward victory (two steps forward, one step back), but one bastion of male-centered tradition remains inviolate: the marriage proposal. Men do the asking, and women wait for them to ask. The vector indicated by this dynamic mimics the sex act itself, with the coin of sexual access being the engagement ring, preferably one with a diamond.
As that famous spy Ian Fleming once said, diamonds are forever, and therefore an appropriate symbol for a promise that is to be kept until either of the parties dies.
Of course, in the mercenary age we live in, this ideal has a more debased variant, namely, the one that says the chunk of concentrated carbon in question is "a girl's best friend." Sure, it permits sexual access, but if you guys want loyalty, get a dog.