Mathematical Virgin
One who has never experienced the joy (or pain) of rigorous proofs in high-school Euclidean geometry or college calculus.
Angel couldn't understand why mathematicians make such a big fuss about rigorously proving mathematical results that are intuitively obvious to her—one should forgive her for her immaturity as she's still a mathematical virgin.
Mathematical Virginity
You lose it when you experience your first Aha! moment—when you feel like parading yourself naked after being struck by an insight, or upon solving a difficult math problem.
Billy lost his mathematical virginity after he found an intuitive proof to the Pythagorean theorem.