Ash her about it

You decide to ask her about that one time after lifting your head up from between her legs. She had just cum, squirting he hot femcum on your face. You were rock hard, she was still going strong, it would make sense to go the extra step. “You want to do it, go the next step? You asked.

“I can’t.” She painted back as she recovered. “I promised” she said and she went down on you to return the favour.

Later she explained that she promised her mother that she wouldn’t lose her virginity until she came of age. Just like your pinkie promise it seemed to hold the same magical bond over her.