if you dummies actually paid attention you'd see that the couple that come in to drink is actually a man and a woman. Yuusuke is not so much jelous as he becomes suddenly aware of their age gap. He gets frustrated because he can see how much an adult saori is in context, with other adults, and that probably makes him feel impotent and, yes, a bit jelous.
Geez, some of yall are really f stupid.