Well, it's like a word beginning with n used to describe black people. Originally that was not an offensive term, it was just a word used to describe them, just as white or English could be used to describe me. Gradually, the attitudes behind this word began to change, as did the meaning of the word.

Same for gay. Used to mean gay people, now it has acquired negative connotations in the same way. Now it has two (three if you count happy) meanings and that's all there is to it.

Many other words now considered racist/homophobic/disablist all started out as regular words that meant no offence at all. Sadly it's the attitudes behind it that matter. If you ask me, no word in itself is offensive, just the attitude behind it.

(Yeah I did this at English Language last year and got an A in my AS levels for it )