Erik's mouth twitches, as it often does when Charles inflicts these bouts of whimsy on him. "To the best of my knowledge I'm not impossible to lie to," he deadpans, taking a hold on Charles' hips that further suggests he is forming a multitude of ideas centered entirely around escalation. It's an interesting sort of nonverbal communication, involving as it does the specific reading of body language without telepathy to bolster it, and that piques his interest, too. "Just difficult."
As he is in ...many, many ways, but the relative mild gentleness of his humor does as much to establish the goodness of their togetherness as anything more strictly physical. Erik doesn't trouble himself to define this kind of closeness as 'safe,' since his perception of safety is fundamentally skewed (it exists for other people, and can border in many unhealthy ways on the infringement of personal freedom; he would certainly have no problem encasing Charles in a literal box if he thought it would keep him from bodily harm), and additionally appears entirely outside of the spectrum of what he thinks of as possible or necessary for himself. He doesn't need safety when he's so imminently capable of self-defense, and the quickest route there is frequently permanent. A person who can't breathe isn't capable of administering another blow.
But if he can make Charles feel safe by kissing him even Erik doesn't have to have it pointed out that that's infinitely better than enclosure in a confined space, so he does that, pushing down on Charles' hips as he does.