Estate planning is indeed centered to a large extent around the preparation of your assets for distribution to your loved ones after your death. But when you are making such plans you will inevitably consider end of life issues as well, and one of these involves the possibility of incapacity. If you were to become unable to make your own medical decisions who would make them in your behalf? This is a question that is relevant to everyone, but it is an especially poignant one for same sex couples.
A living will is one of two advance directives that are a must for same sex couples. With this document you elucidate the medical procedures that you would accept and those that you would prefer to deny should you become unable to communicate these decisions. The point that is central to living wills is usually going to be whether or not you would want to be kept alive through artificial means if you were to fall into a terminal condition.
There is another document that is used to make sure that all of your bases are covered called a health care proxy or durable medical power of attorney. Though the living will does state your health care choices with regard to some contingencies, it is difficult to impossible to address them all in a single document. With the durable medical power of attorney you name an attorney-in-fact, presumably your partner, that you empower to make health care decisions in your behalf involving scenarios that have not been addressed in the living will.