[Only in the plural is a woman likely to be called a guy .] You can say, 'Call Mary and Jane and see if those guys want to come to dinner tonight,' ... but you can't say, 'I wanted to invite Alice, but the guy wasn't answering her calls.' This usage may be related to the tendency to use guy as a kind of perky pseudo-pronoun - nowadays you hear people using a phrase like 'those guys ' to refer to everything from ski boots to PowerPoint slides.

Geoffrey Nunberg