Women are smaller than men. The Miata is smaller than most cars.
Women are smoother and curvier than men. The first couple generations of the Miata didn't have any sharp corners, everything was rounded.
So the car comes across as very feminine. The newer ones are a little tougher looking but still feminine.
But you should drive what you want, nothing wrong with a man driving a feminine car if he wants to, just like there's nothing wrong with Americus driving that manly Wrangler she used to drive .

---