What's her problem?
Her problem is lack of understanding, hence why she was asking. She wanted to learn, and you shut down on her because she was being annoying.
The easiest way would have been to equate it with a woman's habit that she wouldn't want to discuss. The short response to get the understanding would have been "It's a guy thing like your (insert chick thing)" Then she'd get it and not be pissed that you shut down on her.
if she were ready for the longer explanation, guys perspire differently, and our pubic hair situates differently. I have a dry hairy back, and that makes me itchy. If my crotch area is dry and the pubic hairs rub against it, it can be itchy. If it had been oily or wet and subsequently becomes dry, that can cause itchiness. Or sometimes being wet with sweat, water, oil, can cause itchiness too.
Sometimes the under gear can contribute to itchiness too, if the fibers tickle or come in contact with the skin. Laundry detergent residues can cause allergic itchiness. as well.
Yesterday, I had been doing a wet jockstrap photo shoot with several straps, and occasionally stood out on my porch to bask in the feeling of the wet strap on me. Eventually i put a dry swim supporter on (shorts over it) and went to run some errands. When I got to my 2nd stop, I had some SERIOUS itching I had to do.
There are a lot of contributing factors. And depending what's been going on down there can determine the extent of rubbing relief required.