After getting on her knees so that Damon can pound her from behind, Riley orders him to lay on the bed so she can climb on top of him.
Later that day, Riley finds that Damon has been jerking off into her panties and putting them back in her drawer.
They keep going, with Riley falling to her side so that Damon can spoon her from behind as he brings her off again.
When Riley Reed walks in and strips naked, Damon hides in the closet and strokes his stiffie.
Only after her stepbrother has jazzed in her mouth does she realize that she has mistaken his identity.