i firmly believe art expresses his love not only through kissing, but through the mouth itself. oral fixation is putting it lightly. the act of him kissing is one aspect of a greater complex that involves a man using his lips to express his desire, devotion, dedication, determination, and desperation. donaldson's mouth is nothing if not a way for him to give himself (perhaps even control and power) to the other person. the spitting of the gum into both patrick and tashi's hands, the biting of the churro from patrick, kissing tashi's thigh.
art's mouth is also the one thing that gives him away as a character. when he's contemplating, when he's lusting, when he's mad, when he's frustrated. during his kiss with patrick, we see a second mini kiss yes, but we also see his unfiltered desire and even greediness as a character to take through the same mouth that he expertly gives through.
tldr: art can't control his mouth and it's affecting everyone.