Just to put a slightly different spin on all of this... There are times when it would make sense to share a bed. Those situations are incredibly rare.
Making a habit of sleeping together is a definite no-no (as per previous posts). That said, particularly in unplanned situations, it probably isn't worth going too far out of your way to physically separate yourself. If you're both appropriately covered up, the proximity isn't much different from sitting closely on the couch together. Heck, if you're both tired, probably less problematic.
If you are planning the rendezvous, if you're making a habit of it, if you're doing it enough that a casual observer or acquaintance would notice, then you're moving much too close to the fire.