You're a kind soul JohnnyE. Not everyone could make that shift in their mind and be content with the possibilty that their beloved wife has needs you can no longer meet and has to go elsewhere to meet those needs.
You may find you have unexpected emotions when she finds someone. This is very common. Fear, jealousy, envy, sadness, unhappiness - even while acknowledging her joy - are possible. If this happens - and it's not inevitable - allow yourself to feel them. You have the right to feel your feelings, even if they are ugly, not what you expect from yourself and directly contradict what you happily agreed to do with for Sarah. Allowing yourself to feel, even if very painful, is the only way to move beyond those feelings to just feeling the geniune joy I believe you will also experience.
Best of luck!