He's having trouble breathing with Charles' mouth on his cock.
It's not that he's never been here before; he can have anonymous sex of any kind, anytime he pleases, by sitting at any bar looking pretty until someone shows interest - and he has never been shy about taking advantage of their interest.
He has the feeling that he can't breathe because it's Charles' mouth on his cock, and that feeling is not something he is (able) going to deal with right now.
To be fair, Charles is also very good at this, deploying tongue and lips and teeth and suction at all the right moments to bring Erik right up to the brink - then pausing, holding Erik's cock completely still in his warm wet mouth until he can start moving again without sending Erik right off. If he keeps this up Erik suspects he won't be able to stop himself gripping Charles' head in a vise to take control and fuck his mouth rough and hard; but even Charles' teeth against his cock come off as tender and sweet and it makes Erik hesitate even though he can't breathe or think or anything but godcharlesmorefuckcharlescharlescharles and he doesn't know if he's saying this out loud or just thinking it, if he can't breathe he probably can't talk either but he doesn't know, he doesn't, he can't - he's so close -
Then everything stops because Charles' mouth is not on his cock anymore, and Erik is breathing in heavy gasps and his cock is wet and getting cold and not at all pleased about Charles' mouth leaving, and he's just about to say what the fuck Charles when Charles waggles his eyebrows and says with his now so red lips, "You know...if you want, I can use my abilities to access the pleasure center of your brain and create a feedback loop...I've never done it before but I'm sure I could, and I can hold you back from climaxing better too if I have that kind of access, and it would make this so much better, so much more -"
He could probably decipher feedback loop if he could fucking think over his arousal, and he isn't thrilled about hold you back from climaxing, but right at this moment the entire world has narrowed down to getting Charles' mouth back on his cock, every second it isn't is a terribly tragedy, and Charles said it would be better and better is always good and so Erik gasps, "Yes yes do it" and then Charles' mouth is back and it feels like coming home, and Erik doesn't have even a moment to consider the ramifications of that because suddenly he can feel everything Charles is feeling -
His cock in Charles' mouth is velvet and salt and iron -
And his voice in Charles' ears is wordless pleading, gasping, sobbing, and he has never sobbed for it before, never begged for it before -
And Charles is so hard, and was even before he was in Erik's head, has been that hard this whole time because of Erik's moaning and tensing and relaxing and tensing more under his hands and his mouth, Erik's fingers trembling in Charles' hair, the open lust and want and need Erik didn't even know was on his own face, and Erikerikerikerikerik -
And Erik feels Charles come, and it is...glorious and transcendent and Erik has never felt anything approaching it before and neither he can tell has Charles -
And that would be the end of it if Erik could come too - he wants to, he needs to but he can't because Charles is holding him back and so when Charles stops shuddering and returns his attention to what his mouth is meant to be doing to Erik's cock, Erik catches the flood of emotion that races through Charles for just a moment, and he knows: to Charles he is luminous, perfection; to Charles, this is so much more than just sex, it is a prayer and a benediction at the altar of Erik -
He can't breathe again, and this time it's panic, everything he can't look at straight-on thrown in his face where he can't ignore it and he can't come and the pleasure is mounting again and he doesn't want it anymore, it's too late too much too soon -
He needs it to stop but he can't breathe, he can't say it, he isn't mentally together enough to even form the word in his mind and he can't breathe, he can't breathe, he knows all about fucking, he can handle fucking but this isn't it, he can't do this whatever this is and he can't fucking breathe -
And somehow he manages to croak, "Stop," and Charles does, and it's a moment before Erik realizes that he stopped before Erik told him to, that his own panic bled over to Charles' mind at once, that Charles' answering panic to shut their connection down came back over to Erik, that the only reason he was able to ask Charles to stop was because Charles already did and Erik is done thinking about it now.
Charles' cheeks are flushed and his lips are red, and he looks so troubled. "I am so sorry," he murmurs, "that wasn't supposed to come through -"
"Charles," Erik says.
"I'm so sorry, apparently my control needs a little work -"
"Charles," Erik snaps. "Shut up. Give me a minute."
"Okay."
Erik throws his arm over his eyes and breathes in and out deeply, feeling Charles' anxious gaze and urgent need to babble at him. Eventually his heart stops thudding against his ribs and his breathing goes back to normal, and that is when he realizes he is still hard.
Fuck.
He really should kick Charles out of his room and take care of himself, but he (can't) doesn't want to. Against his better judgment, he lowers his arm and looks at Charles looking at him so concerned and miserable and god help him, Erik wants his mouth again.
"So," he says, not thinking about the consequences because that's a place that will only get both of them into trouble, "will you stop looking at me like that and put me out of my misery already? God, Charles."
And then Charles' mouth is back on his cock and it is nearly unbearable even without the feedback loop, but this time Charles isn't playing, isn't teasing, and Erik comes at once, which is all he ever damn well wanted to do without all this other nonsense getting in the way (or so he tells himself).