The first time it happened was during one of their "dates" after her tail brushed against his crotch, leading her almost immediately pulling him into an alley behind the arcade. Sure, it was difficult to keep her teeth from brushing against his dick too much, but it was just what the both of them needed.
From then on, it became a sporadic tradition, either randomly during one of their "dates", or Ellen Joe pulling double duty for her cleaning services. Anytime he'd ask if she doing okay, she'd brush him off, acting like he was an idiot or something to ask such a stupid question, only to hide the blush on her face.
It wasn't her fault that her stupid, handsome, obnoxious, annoying, overbearing, caring, dummy of a Master had such giant gross tasty incredibly addictive cock that needed cleaning every once in a while ... which just so happens to be whenever they're alone with no one to see her act like this. Maybe it's just a part of her dna, like how sharks go crazy when blood is in the water, she goes a little ... weird, whenever she smells his cockmusk in the air.
Just puts her straight into heat~