You can't guarantee that she will keep her vow, you can only guarantee that you will keep yours. She can change her mind any second and there's nothing you can do about it.
Marriage is not a commitment. Marriage is an at-will arraignment, as the divorce rate will evidence. Americans have stopped acting 'in good faith' to each-other in all things. As such there's little point in marrying at all and gays will soon find this out.