DWW: Josh and Grace, the Girl Who Wouldnt Spend the Night

Josh is REALLY into Grace. They've gone from friends, to dating, to now full-on romantic partners. They've done pretty much all the normal "couple stuff" together, including the "bedroom stuff". But for some reason, she REFUSES to spend the night with him. It's not like they haven't seen each other naked! So why does she refuse to sleep overnight with him? There's definitely a reason, and it's NOT what we thought!