In my world, broken sexual promises stain a person's genitals blue, but I cannot decide how casual sex would survive at all

The stain appears only when someone makes a promise to be monogamous, or when they boast about the orgasms they will deliver. It starts as a pale line on one ball or labia flap, then spreads if the promise is ignored. No priest or judge is involved; the phenomenon just... happens.

My problem is ordinary dirty talk. “I’ll cook you breakfast in bed tomorrow, so how do you like your eggs, you sexy thing” “I’ll blow out your back.” “I swear this rash is not contagious.” Would sex become painfully literal, or would hook-up cultures invent phrases that clearly mean “this is intention, not a vow”?

One region already covers their genitals while speaking about future plans to fuck. Another considers uncovered genitals a sign of trust. I’m tempted to make children learn promise-safe sex ed everyday at school, but that feels too tidy. What messy pick-up lines, etiquette or loopholes would grow around this?