I was at a wedding on the weekend and had the privilege of being one of the groomsmen, and also one of the official witnesses to the vows. So I listened to what they said to each other in order that when I signed as a witness, I was actively and intentionally signing it. One of the promises that they made to each other was to devote themselves to each other, forsaking all others for as long as they would both live.
I've been thinking about this phrase for some time and have come to the conclusion that it is one of the most unnatural and amazing things to promise anyone else. Natural mammalian urges are quite selfish in almost every way and are, as some folks theorise, the effect of ensuring that one's own genes are passed on. Eating and procreating, being among the strongest drives, easily fit into this category and can be used as a backdrop to much of human behaviour. Compare this to the idea of selfless monogamy, in which one person commits to another that they will take control over those drives in all areas and ensure that the needs of one's partner are higher than all others, including one's own.
The freedom in nature to eat or shag anything else that comes along is given up for the sake of the new partnership. It requires a choice by each partner to deny those urges and freedoms so that the partnership can succeed. This choice is the action of love, the existentiality of love. Without this choice, love does not exist and is reduced to little more than a greeting card sentiment. To love is to forsake. To forsake all others is to forsake self-gratification. Even though the phrase we most associate with weddings is "I do," it means much less if we forget that this doing requires "forsaking all others."