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.
"Guys" can be used in both a male-gender-specific sense (for which the female-gender-specific rough equivalent is "gals"), or in a gender-neutral sense; there is no structural way to distinguish the uses. Like most places in English where a word has both a male-gender-specific use and a gender-neutral use, the latter is somewhat controversial with certain groups, who prefer unambiguously gender-neutral terms rather than dual-use terms that can either be male-gender-specific and gender-neutral.