Charles doesn't think of safety in physical terms, which is one of those things he knows can't be explained to Erik, at least not where Erik now sits. It would seem insane at best and trivializing at worst, when Erik never had physical safety to take for granted in the way that Charles did as a child. But Charles did have it, so as an adult he is capable of putting safety elsewhere. It lives in his connections to other people now, like his trust in Erik's deepness of feeling for him.
It also doesn't hurt his sense of security that he knows Erik would protect the both of them from absolutely anything that threatened this moment, and Charles is no slouch himself when it comes to self-defense.
"Oh, I was being ridiculous. I'd be too nervous to lie to you. I'm so much putty in your hands." Hands that Charles moves with in supple, sinuous fashion as Erik pushes down, Charles' mouth parting in a tiny 'oh'.
"Besides," Charles kisses him again, sweetly, and tips their foreheads together, "I can't imagine the trouble I'd be in."
There is something in the way that Charles nips at Erik's lip next that suggests he knows exactly what kind of trouble he'd be in, and he doesn't mind in the slightest. It's comforting to glide on the smoothness of Erik's broad chest, different than his own slender fitness that's so rarely remarked on or visible, and know that this is still an evolving, vital thing between them.