Erik is right to anticipate a blanching on Charles' part at the association that the joke makes, and it's only mostly calmed by how tightly Erik holds his hands. What he does to rid himself of the rest of it is push the covers aside and coming to sit nestled against Erik's side, resting his head on Erik's shoulder as he readjusts their held hands for the new position.
"It's...a mix of things. I think if we ever do tell anyone, and I'd like to tell Raven eventually, she won't be upset--then you're my significant other." Charles...has thought about this, obviously, and the things that would make Erik the least uncomfortable. He's quite good at compromise, as Erik will know, and besides, it isn't as if it's very hard to fill those two words with as much love as the next one. "But between us...I'd like you to be my lover, Erik."
He blushes softly at saying it, squeezing Erik's hand: "I know it might sound a bit silly, but that's how I think of you. We can change it if you ever think of something you like more. I...I care about you. Very much."
Erik might be able to hear the catch where another word was what Charles wanted to say, and he might not, but Charles is aware of it. In part he keeps it because he's shy around Erik like he isn't with anyone else, but then it's also that he just doesn't know how it will be taken.