Merlin was initially determined not to have sex with Arthur.
Not that he didn't want to have sex with Arthur, in fact he very much wanted to have sex with Arthur, but Arthur was already a spoiled prat as it was, used to getting his way, and if Merlin jumped into bed with him after they'd been trying this relationship thing for only a week then what kind of message did that send? No, Merlin was going to do this properly. He was going to hold out until Arthur had proven his devotion and willingness to make this work and, and, and...
And something.
It was a little hard to think when Arthur had him pinned up against the wall like that.
"We—we should—don't stop, don't stop," he panted, his protest turning into a plea as Arthur continued to rock their hips together. Honestly, they were still fully dressed, it shouldn't be this mind-melting. But instead of pushing Arthur back and insisting that they wait, like a normal person, Merlin clutched at Arthur's hair and a handful of his shirt and crushed them together instead, letting Arthur suck a vicious bruise right at his collarbone.
"Bloody laces," Arthur growled, pulling away to focus on getting their pants off.
Merlin squeaked and tried to bat the prince's hand away. "We're in a corridor!" He hissed.
Arthur looked up at him and honest-to-God growled, his eyes such a dark blue that they were almost black. Merlin gulped, a shiver rushing through him at just how possessive Arthur looked. The prat, of course, took advantage of this and finished undoing their pants, and then he was slipping a hand inside Merlin's trousers and yes, all right, maybe objections could wait until his brain started working again.
It was, all things considered, rather unsophisticated. When he'd pictured this he'd usually imagined it being on the bed in Arthur's room, with soft light filtering in from somewhere (the window? candles?) and lots of pillows for propping things up and getting at just the right angle. Instead he wound up with his legs wrapped like a choking vine around Arthur's waist, his back scraping up against the wall (that would bruise tomorrow, he just knew it), sobbing into Arthur's mouth as the prince did absolutely criminal things with his tongue.
It was glorious.
Really, his imagination didn't measure up at all.
When he was finished, panting into Arthur's (still clothed) shoulder, he had a sneaking suspicion he was ruined for anyone else.
Then Arthur mouthed against the bolt of his jaw and growled, "We are going back to my room," in that stupid spoiled of-course-I'm-going-to-get-my-way voice and Merlin's suspicion became a certainty.