Rei uses "douteippoi" meaning "like a (male) virgin" so I think she's just a bit disappointed at Mitsuki's constant shyness/passiveness over every little thing. Since Rei's dated at least one person before, I imagine it'd be a bit tiring to have to go through the "We're holding hands!!!" phase all over again. That said, they've only just started dating, so I think it'll all work out eventually.