Why do girls think I'm taken?

A couple of the girls that I talk to assume that I have a girlfriend and said that's why they never tried to further anything in our friendship. What could I be doing that gives off the vibe that I'm taken? I want to appear single like I actually am. I've been single for 3 years now. I've been told I have the traits of a great boyfriend, so maybe those show even though I am single. All answers are appreciated. Thanks :)
Why do girls think I'm taken?
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