You're absolutely right, sooner. You cannot make the assumption a woman is having an affair just because she loses her desire. In fact I used to test myself and try to fantasize about other men to see if I could generate some sense of desire. Zippo. Nada.
Now on the subject of needs getting met. I'm actually suggesting eliminating the concept of "getting needs met" as a central core to making a relationship work. I'm suggesting focusing on your enjoyment of the other person. My h and I have had discussions about getting needs met throughout our marriage. Looking back on them, it was mostly me talking about my needs. They seem rather monotonous and similar now after our more recent conversations about our relationship. Our recent conversations have focused more on enjoying each other and talking about ideas, thoughts, plans. We do things for each other now not to just fulfill a need the other person has, but because we just love doing it for the other.
This time around, I really have a much stronger sense of real, significant change in our relationship compared to my feelings after the previous, old, boring conversations about needs (my needs.) Now that my eyes are really open to seeing my h again, I'm learning so much from him. (And that's really sexy!) He says he doesn't see me as a "fulfiller of his needs." He sees me as his partner in life. I'm working as hard as I can to be more like him in this way.
He just came home and read what I was writing. He says he just loves me and not to stress over it too much.