There are certain things that normal politeness requires you NOT to do when things are starting to get hot and steamy in the bedroom. Sherlock Holmes, genius and all-round know-it-all, ought to know this. He ought to know that when you are hovering over one increasingly red and panting John Watson that is currently very pre-occupied with bed-related activities you ought to stay focused. Concentrate on the task at hand.
Look keen.
You are NOT supposed to very nearly leap at your mobile phone as it gives a merry chirp from the sidetable.

Even though Sherlock is wise enough not to actually bodily leap at the phone, his gaze and his focus do. Due to John's rather awkward position he, of course, notices this at once. At least as soon as Sherlock lacks his grip for a nano-bit, which causes the small doctor to get badly pinched.
"Ouch! Sherlock!"
"Hmmmm..?"
"Focus, goddammit!"
So Sherlock focuses and gets a firmer grip on the bed-frame. His gaze is locked at the phone, however. Again: John notices. Of course he bloody well does. Especially when Sherlock tries to multitask, and stretches one long arm over towards the bedtable while at the same time half-heartedly trying to do things that will please John. John is not pleased – he is pinched.
"I swear to God", he pants, "If you try to answer that call I will kill you. Geddit?"
"But Jooohn! It might be important. It might be Lestrade! It might be Mycroft. My queen might need me!"
"Right now I need you to fucking concentrate on getting THIS plug to fit THIS hole and nothing else. Right?"
That was the Captain Watson voice. For some reason or another Sherlock's usual defences did not work against the voice of Captain Watson. He concentrated on the plug and the hole, as ordered.

At least for a minute. Then his thoughts started to wander again. This had all been quite interesting to begin with. It was fascinating to study John trying to be all dominant and manly while getting increasingly sweatier, swearier and hotter. Seen from a logical and analytical point of view, these activities were obviously extremely inefficient, but they were nevertheless an interesting study of human behaviour. He also got to do very interesting studies of John's anatomy as he squirmed and squiggled beneath him. John groaned. Sherlock sighed.
"Is this going to take much longer? My arms are getting numb."
"We will do this until I'm done. No matter how long that takes", John panted between gritted teeth.
"This is tedious."
"Then try to put some effort into it!"

Sherlock scowled but he did put some effort into it. He gripped the frame until his knuckles turned white and then he shoved with all his might. John nearly screamed in triumph.
"That might just work! Keep still, keep still!"
"Really, if we could just do what I suggested from the beginning and just remove the pillows and flip..."
"Shut up. I know what I'm doing. Lift... Lift...Ah! And to the right. Yes! No, no... To the left."
Again, Sherlock did as he was told. John was the expert after all, there was no point trying to deny it. Before the doctor came in to his life Sherlock would never even contemplate doing anything remotely like the activity they were engaged in now. And his knees would be very happy to return to that state of ignorance, he though bitterly while long legs started to shake. John grunted again.
"Try to rock back and forth for a bit", he ordered.
The bedframe squealed in protest, Sherlock huffed and John groaned. Sweat was dripping into his eyes. Then all of a sudden, they hit the perfect spot.
"There! There! Keep still. Ooh, that's perfect. Don't move, don't you dare move... Ah ha ha ha!"
Some vigorous screwing commenced. Sherlock straightened his back and held his breath. And then John gave a cry of triumph and slumped back.
"Ah ha ha! Bloody brilliant. I'm done. You can relax now, Sherlock."

With a sigh of relief Sherlock let go of the bedframe and rubbed his aching back. He was actually sweating. John's cheeks were flustered and his hair wild as he wriggled out from underneath the bed, wielding his screwdriver like a weapon.
"No Watson has ever been defeated by an IKEA bed before, and neither shall they ever be!" he beamed and sauntered over to the detective. For someone so small he could actually radiate quite a large amount of manliness and self-assured power.
"No. Because next time we will buy a bed from Harrods, like I wanted from the start."
"Next time", John breathed in his ear, "You will not set fire to our bed while I'm at work."
"It was an accident." Sherlock had planned a more eloquent retort than that, but he lost his train of thought when he realised that the thing prodding him in the thigh was not the handle of a screwdriver.
"Hmmm... Wanna test how much pressure the new plugs can take?"
"Oh God, yes."

This time Sherlock did mind his manners. He did his part of the work until John was quite quite done.