For a different point of view see http://www.cs.virginia.edu/~evans/cs655/readings/purity.html

There are two related but subtly different uses here. A word like 'he' is a default, which can lead to problematic implications. A word like 'guy', to some people, is a true neutral, implying gender no more than the word 'somebody'.

Even if we were talking about 'he', your link would be out of place. Calling out the use of a gendered default as bad is not in fact another point of view on whether it is the default.

I have never heard of a female being addressed as "guy". Phrases such as "hey guy", "what's up guy" and so on have always been directed to a male.

Sometimes I will use it in a conversation when referring to a third party unknown to the person I am talking to and the subject's gender has no bearing (like in my first comment).

For example, "Oh, I know a guy that knows X," "I know a guy that is a fan of X" or in a less positive manner "That guy just cut me off [on the freeway]!" (since you rarely see who it might be and their gender doesn't matter)

I could say "dude" as well, but sometimes dude might be too informal. I could reword it with a number of other pronouns, but it depends on context and whom one is talking to.

"Hey, guys!" in reference to a mixed group is perfectly acceptable. "That guy" in reference to someone you do not know the gender of is also perfectly acceptable. Directly referring to a known female as "guy" is less so.

In phrases like those the word "dude" is more common than "guy", and it is frequently directed at women.