It could be a sign. I think something "physical" and "obvious" like losing sexual attraction would be easier for me to deal with than something more personal or emotional. And, to a certain extent, it gets one off the hook. . . "It's weird because sometimes I'm still hoping to make the relationship work." It seems you can't change how you feel, so. . . oh well, no point in trying anymore.

Have you ever not done anything with the sexual aspect of the relationship until you got to be friends/know one another really well? That might be interesting to try, spend a few months talking/getting to know one another well and get the relationship well established on other grounds before adding the sexual. It looks like there's a "pattern" so maybe think of some way to interrupt that pattern and replace with another so the sexual attraction on/off switch doesn't function the way it is now?