Cheating on your girlfriend, being a pervert's plaything, and doing it for the sake of your sensei is, like, just enjoying your youth, no need to feel guilty as long as everyone's happy, huh.
And of course she'd take issue with his sudden boost in virility that made him power through the shopping routine in a way no other MC could pull off. That's an odd case of turning into a losing girl now that she couldn't have her regular dose of Toameium.