What does that mean, when a guy ask for your cup size?

My guy friend asked for my cup size recently. And i was shocked and said no, its personal. And he kept asking me until i told him. And when he heard the answer, the first question was "Have you ever told your size to any of your ex's", i said no, coz they never asked. And he seemed so happy and said, "so, i am the only guy knows it".
What does that mean, when a guy ask for your cup size?
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