Dusting off something from Cecil’s achives - the article relating to Does passionate kissing cause your chewing gum to disintegrate? has another possible explanation.
It is my experience that phlegm will cause gum to break into small pieces. I have learned this from having coughed up some of the stuff whilst chewing.
Which, if you tie this back to the original question of having gum disintegrate during passionate moments with your partner, leaves the observation that they were swapping more than spit… Ewwww… 
Is phlegm chemically related to saliva? Could there be a connection that way?