Here's the thing Charles doesn't quite get about Erik:
(well, not the only thing -- Erik tends to have hidden pockets of enigmatic darkness that Charles sometimes runs into and sometimes doesn't -- but it's one of the things that frustrates him the most)
--Erik doesn't want sex.
It's not that he doesn't want Charles. Even without telepathic backup, the signs are clear enough; he finds every excuse possible to be touching Charles, especially when they're alone. He likes kissing, likes running his hands through Charles's hair, likes kneading the tension out of Charles's neck and shoulders and back until Charles is a boneless heap of purring relaxation, likes Charles running his hands over his body (unless the touch is light enough to tickle, which leads to him swatting Charles's hands away with a low growl). And when he lets Charles into his mind, the surface thoughts are full of want and love and need.
But every time Charles tries to take things farther, Erik's mind shutters closed and he finds some excuse to pull away.
It is beyond frustrating, and Charles doesn't understand it at all.
"Fuck me," Charles begs, half out of his mind with lust and desperation; "please, please, God, Erik, fuck me, I want you, I need you--" and he knows his thoughts are leaking into Erik's, his need to be held down and pushed open and filled and used.
"Why?" Erik murmurs, his hand on Charles's hip but not moving, and Charles wants to howl in frustration.
"Just please please, Erik, I need," and then Erik's hand is moving, thank God, to Charles's cock, hard and leaking with pre-come, and Charles thrusts helplessly into the touch, wanting more but knowing Erik won't give it to him.
"You are a very strange person," Erik murmurs, running his fingers through Charles's hair.
"No," Charles says, "I'm normal," and for a moment there's silence and then the absurdity of the wording hits him and he's laughing, Erik's smiling, Charles's mind is full of love and amusement and gentle tolerance. "Sex is a biological imperative necessary for breeding--"
"--and it's also fun and pleasurable and honestly you don't know what you're missing out on, do you?"
Erik raises an eyebrow and says, "Actually, I've gotten quite a mindful from you."
Charles rolls over so that he's lying on top of Erik, blanketing him. "Then why?" he begs.
"Because..." Erik tilts his head, considering. "Because I'm not like you."
Charles is so full of need that Erik agrees, although his expression is skeptical, his mind humming with doubt. "Once," he says.
"Once," Charles promises, "unless you change your mind."
Erik shakes his head ruefully.
Erik strokes himself to hardness, watching Charles watch him. "May I?" Charles asks, because he wants to show Erik, see if he can get Erik to understand what it means.
Erik nods, and Charles opens his mind. He shows Erik what he sees, but he can also feel what Erik feels, a rough relentless grip on himself that is no more (and no less) pleasurable than scratching an itch.
As a counter to that, Charles opens one of the packets of lubricant and gets two fingers slick so that he can open himself for Erik. He sees what Erik sees, Charles contorting enough to enter himself, body arched with pleasure, and hears what Erik thinks, but he also sends back what he's feeling, the pleasure and anticipation and stretch.
"How do you want to do this?" Erik asks him quietly, once everything's ready, and Charles just pulls his knees up to his chest, spreading as much as possible. "Don't care just do it do it," he pants, and Erik gives a wry twist of his lips before he settles himself against Charles, blunt tip of his cock pressing against Charles's entrance.
"Let me know if I hurt you."
Charles makes a frustrated keening noise; he doesn't care about hurt, doesn't care about anything but having Erik inside him. "Please," he says desperately, and then Erik is sliding into him and filling him, claiming him, melding with him.
"You really do like this," Erik murmurs, stroking hair out of Charles's eyes. His voice is full of wonder and bemusement.
Unable to trust his voice, unsure that words could be enough, Charles just reflects back what he feels, the sheer joy and need, the burn that isn't so much pain as pleasure.
Erik reaches for Charles's hands, clasping them together, and then begins to move, and oh, it is so utterly perfect, so utterly beyond what Charles had imagined, that he feels like he's going to spontaneously combust. Erik might be holding back, but there is enough strength behind each of his thrusts to be wonderful, glorious, and Charles soars, overcome with sensation, his mind bleeding the feelings of so-good so-right yes-Erik-please into Erik's mind.
Beneath that, he is dimly aware of the thoughts he's picking up from Erik, baffled pride that he can do this to Charles, a hint of wistfulness, and contentment that has everything to do with pleasing Charles and nothing at all to do with fucking him.
Charles is sweaty, sated, exhausted. "Do you understand?" he asks Erik.
"Mmm." Erik presses a kiss to the nape of Charles's neck. "No, not really."
"You are so weird," Charles murmurs drowsily, and feels Erik chuckle.
"No more than you, my friend."