So a dream, this could be good or bad... bad because he may wake up in the morning, having missed the chance of continue a shower scene, good because with this nightmare/dream he admited he wants his mom all for himself, I hope he wakes up right now and Sarah is still in the shower, Aaron has to act!
If it is not a dream, then Aaron is half asleep and imagining things, there's no chance they will give us a David/Sarah scene like that (unless they give us a flashback later) but again, since Aaron can't even open the door, it's a dream or an hallucination.