I can already hear the "OH YEAH~" sound effect during Rintarou's fantasies.
He should've realized that what he saw wasn't her bra, it was the top of that string bikini he's always fantasized seeing on her.
She was actually paying attention to when he said "string bikini" and she actually showed it to him. Bold move.
Now buy it and have a "fashion session" round 2 at home like actual lovers