You have also said if some one said just dirty bitch on its own, you would think they were being racist. That's not what would immediately spring to my mind, sorry. I would immediately think it meant someone who slept around, or was smelly perhaps. We are not all racist, and not all words are racist, but see that in your mind they obviously are..
Nice. Try. A stupendous Oscar-winning attempt at a "you just see racism everywhere"! V. Impressed and very much brightening my afternoon. 
Except not what I said, is it.
I reproduce it here:
If you told me that "dirty, etc" it had been used to a person, I'd say it was probably a jibe at someone's ethnicity.
I would have thought that "dirty, etc..." was very obvious shorthand for DBM, as it's not the nicest string of words to type out repeatedly...
I took it that he meant dog in the sense of a woman who sleeps around. Again, to be clear here in case you think otherwise, in no way is it ok to say that sort of insult to anyone.
And the reason you didn't say, "I think it could be intended as a sexual slur, rather than racist" earlier is? You seem to have been putting a very positive spin on your results...
Me, if I'd asked around and decided it was unlikely the boy was being racist and just intentionally misogynistic about a 14-year-old sleeping around, I'd mention that, too! Why?
Why are you so invested in only mentioning the most positive interpretations?!