Charles smiles in a way that looks much more like a wince--he should be glad, he knows, that Erik has told him that much, and he is. He hadn't really thought it was meaningless to Erik, but he had wanted to offer a safe way out of it for him. Before Charles could inflict or allow whatever nebulous hurt is poised to scar Erik so badly. But Charles won't and can't allow that to come to pass.
"Yes, I know. I wanted to save face for both of us just--in case you changed your mind." Charles reversed the hold on one of his hands to draw Erik's fingers to his mouth, where he kisses them, and then holds his hand to his mouth for a long time with half-closed eyes. He doesn't know if he imagines that Erik smells like metal from such a long time spent being sure that's what he had to smell like, or if it's true. Either way it's the subtler things that fascinate him now, like the traces of soap from washing hands, and the rough texture of Erik's knuckles on his lips.
"I won't tell anyone about us, you don't need to worry about that. You're not my first boyfr--" Charles wince and tiny ribbon of laughter are less pained, this time, and he looks up at Erik with a glimmer of hope where he had been trying to discipline it away before. "Boyfriend. It sounds so very freshman year. You're hardly a boy, although you'll always be my friend. But it's not an adequate word. Significant other? Lover? My Erik? What do you like?"
Charles wants a name for them. At the bottom of his anxiety that's the problem. He wants to know where he stands and what things are, and this is the gentlest way to tell Erik that, as far as he knows. It makes it a choice, even if the choice is not to put words to things--because Charles thinks he could live with that, as long as some thought is given to it by both of them. The important thing to name is not what they might call each other, but the bond between them. To name a thing makes it more real, and Charles wants this to be as real as it can be without being made of iron.