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.
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.