Nor has it happened to me, and I have to say... I don't think it has anything to do with non-monogamy. This kind of thing happens to lots of couples. Some people are soul mates who will be together forever. Others are not. Of those who are, it still takes boat loads of hard work to keep the relationship full of fire. It doesn't come easily.
Did he actually say that he is not attracted to you because he knows your body too well and he prefers new people? Or is that the story you're narrating based on the fact that your relationship has dwindled?
In our relationship, whenever my husband gets attracted to other girls, it spices up our relationship. The things that take the fire out are always, without exception, problems within the relationship itself.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker