I think it's indicative of higher interest. Imo it's comes down to the nature associated with the mouth in general vs other body parts. To a more or less degree we nourish ourselves through our mouthes, we convey meaning through the language that comes out of our mouthes, hell - when you're extremely out of breath you in/exhale life giving air through our mouthes. To give a bj is to then give your tree of life - so to say - an analogous association with these things; albeit more often then not, unconsciously.

I would say it's indicative of a man who has inspired that "genuine desire" spoken about here. She wants you so bad she values your shaft's pleasure through the part of her body she eats, drinks, breathes, and speaks.

Just a theory