I'd like to argue that oxmen isn't actually a slur. Master dennet says it with admiration, and during trespasser even dorian will say it in the deep roads. Probably just people who know "qunari" isn't the technical term for all of them?
Actually that one is a slur.
Master Dennet also says Halla-Rider to an elf and Halla-Rider is still considered a slur by Lavellan. Master Dennet is quite racist despite his well-intentions, much like most Andrastians these slurs are normalized for him (along with Hedge Mages and Rabbits).
While Dorian is hardly a good source either, seeing as to him racism against Qunari is actually a culture thing. He is instilled with that racism and thinks nothing of it, especially since as I said in Tevinter it is not uncommon for Qunari slaves to be treated like Oxen.
I will say that sometimes there are cases when Qunari will want to be referred to as Ox-men simply because it induces a fear of them, it perpetuates the idea that they are mindless beasts who can crush those in their way. Think of The Iron Bull and how he prefers the article as it makes him seem like a thing, a tool; which he utilizes.
So Ox Men isn’t really something a Qunari wants to be called, but like everything with the Qunari they are going to use it to their advantage.