To elaborate a bit on why it's considered controversial, I recommend this article by Douglas Hofstadter, from Metamagical Themas: https://books.google.com/books?id=o8jzWF7rD6oC&pg=PA152#v=on... . It's a pretty rigorous treatment of a pretty weird issue, and it's got pictures!

To roughly paraphrase: If "guy" means "man or woman", and "gal"[] means "woman", the two words we actually have are "person" and "woman". The implication of that should be unsettling to anyone who doesn't truly believe that women are a distinct group from the general, male, population.

[] Or whatever.