"I've got an ulcer in my mouth so you better kiss me."
porn logic.
Lol I agree
I've seen the joke in erotic webcomics and romcoms from USA/Europe/Canada, and seen it a lot of times, so it's not Japanese-specific.
A: "The tea's hot, I burned my tongue!"
B: "What can I do?"
A: "You need to apply saliva!"
B: "Saliva!? Where in the world am I going to find...... oh right, I know!!!!!"
And voilà, tongue-kissing ensues.
