Sex is the best indicator for a woman's attraction level. If she doesn't want to fuck you she isn't attracted to you anymore and the chance that she already left the relationship mentally and is just staying for convenience, preparing to break up or is/will be cheating increases.

To clear some things up. I am talking about a full stop here. No sex at all. I ain't talking about special circumstances like health problems or the occasional "I am not in the mood" after a long day at work. If a woman is never "in the mood" or flat out denies sex without an explanation/excuse regularly, then it becomes clear that the attraction is gone.

The only factor which could indicate that the attraction is still there and the sex stopped for other reasons, is if the affection and behaviour stays the same. Then there is probably some other kind of problem (like health) which needs to be addressed before the sex can continue.