Labels.

We've all got labels.  By themselves they're neither good nor bad.  Its when we use those labels to limit or define those we associate with or see as human that they become a problem. 
For example, back in the dark ages when I was going to SWT to get my second degree, I used to hang out in a coffee shop.  Shocking, I know.  But this place had a lot of Johns that hung out there (the inside joke was of course that with that many Johns in one place it had to be a front for a house of ill repute).  There was Gun John - he owned a gun shop, English John - not English, but an English Major, Jonathan - name and label all in one, if you said Jonathan, everyone knew of whom you spoke, John who worked there, ParmesJohn - a play on his last name and a form of cheese, and so forth and so on.  But we had good reason to attach a label to a person - it helped keep things straight in the mind of the person you were speaking with.  Saying "So, John and I hung out at Sewell Park today" left a lot of room for Johns, and clarification helped.
Then, of course, there is self labeling - what you call yourself in the wee dark hours of the morning.  Some people share this and some people don't - its highly personal.  I can call myself anything I want - if I choose to tell people I'm a high elf from Melbionie, that's my choice.  If they choose to not believe me, that's theirs.  Now, polite individuals will, at least to my face, honor that choice.  Less than polite individuals won't.  They might even be so rude as to ask for proof, or try to tell me how I'm incorrect - the fact that Melbionie is a fictional construct might be bandied about, for example.  Folks who see people as individuals rather than labels will accept it and move on.  Its your choice to be what you want to be, and unless you want to take me with you to recover Stormbringer, its not really any skin off of my nose how you identify yourself.  Just don't get bent out of shape over how I choose to identify - especially if it doesn't fit within the meme you have self created.  Which seems to be the problem today - rather than accepting people for who and what they are, most of us want to pigeon hole them into slots that fit our picture, not theirs - for example, we see a male and a female doing things together and automatically assume they are either dating or married.  Even if we later find out they are married (to other people) or are of differing sexual preferences, the first thought that pops to mind in most cases is "ah, a couple".  The thing is we (as humans) seem to be wired this way - we look for patterns and try to make things fit.  Which if you think about it says a lot about things like the Bermuda Triangle. 
What can we do about it?  Recognize the bias and try to work around it.  Treat people as people, not as a peg to be put into a hole.