Why must the couple sharing the bench with me have to kiss as if they will suffocate if they don't french kiss for one minute?

Let's analyze this scenario. Assume the mouth to be a spherical pressure vessel with radius
r and thickness
t. Due to the sucking action of kissing, the vessel experiences a negative gage pressure
p. The stress experienced by the wall of the mouth will be
So, as the kissing gets vigourous, the magnitude of the stress increases and thus when yield strength is reached, your mouth will rupture. People with small mouth (small
r) and thick skin (big
t) will be able to kiss more vigourously.
But most of the time, people with big mouth also have thick skin.