I try to use "they" when I do not know the gender of a person, partially just because I have run into less trouble using "they" than I have using default male, but also because "they" makes more sense to me when the gender is unknown. To me it would be like calling a object I do not know the name of "table", a noun for a specific object, rather than calling the unknown object "thing" or something on those lines. I do mess up sometimes and use default male, sometimes because I'm still in what I call my "writing mode" where I use the terms and language I would use in a paper rather than a conversation (for those wondering, atleast I was taught default male as "proper" in writing, no idea if that is limited to U.S. or is true in other places, but most teachers do count off) or I'm just not paying enough attention to notice.
I also think "it" is an offensive term for a human, as "it" is a term used for non-human objects and tends to imply that the speaker does not think of the person as human. Now I'm not going to correct anyone unless I know them and know they are simply saying the "wrong" term for the same reasons I normally do rather than them making a point to say it one way or another, as that is just a waste of effort in my experience to try and correct them.
