She seals it with magic

Tonks licks her now-sticky lips with a soft moan of satisfaction, but Ginny’s gaping mouth still poses a messy problem for the futa witch. Tonks dips her tongue back into Ginny’s mouth before it wells up with goo, and she traces her tongue around the inside Ginny’s lips. As she pulls away, she firmly pinches the poor redhead’s mouth closed. Instantly, Ginny’s lips merge together into a smooth and featureless stretch of skin, sealing shut permanently.

Ginny’s eyes widen as she feels her lips simply vanish. She tries to speak but her mouth has been ruined by Tonks’ casual play, and she can only make muffled grunts of distress. Tonks giggles and wipes the poor girl’s face, watching her struggle and her cheeks slowly bulge around the still-leaking penis.