It has nothing to do with Victorian "morality," which is almost entirely mythic. (Start reading Matthew Sweet's Inventing the Victorians and continue reading the related literature.)

Generally, however, Victorian husband and wife shared a bed unless the wife was pregnant (which had nothing to do with morality, and everything to do with the perceived health of the baby), or unless their relationship had deteriorated to the point where they could no longer stand each other (like many modern couples). I don't have a specific source for this, but it's intensely common in all of the fiction of the era.

So, without having read "The Yellow Wallpaper," either their relationship has deteriorated or the wife is pregnant. This is an fairly uncommon state of being.