What Nyanya said. Remember how long it took him to admit that he might have feelings for a guy? I think he's just the kind of dude who would... freak out when something he wasn't expecting comes up.

[ like when yu confessed. when he thought that yu and karkat were dating. ]

You're not going to lose him. He'll be back, once he's done having a crisis about how to respond.

[ he wants to add that no one would willingly lose yu, but that's... too mushy. ]