For those that are straight, you (straight females) have never thought how pretty other female is without being sexually attracted to her? I think that's what it is.
Sometimes I would hear something from purely straight guy that "if he was gay or female, he would do it with that famous handsome actor." I mean that is coming from a guy that loves only women. That kinda made me think that sliding scale does exist.
But why does labels exist in our society? I think it's just how our brain were set up as. Watch how parent teach babies to identify food. Point at red odd shape and say it's "apple." Baby learn to see that as "food" then "apple" then later on, as a "fruit." To make thing simpler, but of course not always for the best. Without labels, we may get confused.