Turd snout
With respect to human sexuality and arousal, a point is often reached that limits both the willingness and accessibility to advance into more adventurous, often uncharted territory; the surprise digital detection of a rectal 'occupant' serves as a cautious reminder to proceed with caution unless alternative preparations can be arranged.
She was so hot and seemed to want more, so I slipped my finger in and, damn, a turd snout ruined everything...amateur!