If you run into a gate mouth first, you’re going to have a problem. Smashing your mouth into a gate is going to hurt. A lot. The problem you’re now in is one of getting to a hospital and having your mouth sewn back on.
The reason you ran mouth first into this gate was because you were being chased by a monster (もん). A monster in and of itself is a big enough problem as is, let alone smashing your mouth into a gate. You poor, hapless soul.