I'm just wondering whether the same feeling of trepidation would exist if the F/O left the flight deck for an extended time and left the Captain on the flight deck by him/herself?
In the mob I fly for both F/Os and Captains are trained and checked to the same standard and both hold a Command endorsement on the aircraft.
In a multi-crew aircraft sometimes the F/O and a S/O can be left on duty on the flight deck while the Captain is fast asleep in the crew rest, which may or may not be anywhere the flight deck depending on the particular airline and aircraft configuration.
Just because a crew member is an F/O doesn't necessarily mean he's inexperienced or less capable it usually means his chance for command training hasn't eventuated yet.
Personally if I want to go into the cabin for other than just a short break I'll check how my offsider is feeling and discuss any operational aspects that may arise though if the weather is doubtful or you're about to enter a busy ATC environment I'd delay until things settle down.
Regards,
BH.