What does it mean to use a slur?
a slur is a word with derogatory meaning which gets its negative connotation from the 'undesirability' of a group of people. It's a word used to mean 'bad' because it refers to a group of people who are labeled 'bad' by society. It doesn't matter if it is being used against a person or not; it is still a slur. Slurs still cause harm when they are used about situations or actions, because they're still reinforcing the idea that a certain group of people is less worthy. The logic train is mostly subconscious, but it works like this: someone says "this situation is gay" and the implication is "this situation is bad like gay people are bad." If you said a situation was shitty, that would have no meaning if people didn't assume that shit was something unpleasant and unwanted.
So, if someone calls a situation 'gay' or an action 'st*pid' or a thing 'cr*zy' when they find the situation/action/thing upsetting and undesirable, they're (unintentionally) implying that anyone who is put in those categories is also upsetting and undesirable just for existing. When someone uses the word 'b*tching' to mean 'complaining' they are reinforcing misogyny. When someone uses the word 'id*ocy' to mean 'selfish evil' they are reinforcing ableism. (I see that last one so often in people speaking against oppression, and it fucking burns) Etc.
So please, don't call your actions st*pid, or use other slurs to describe things, situations, or actions. Usually what you mean is "illogical" or "thoughtless" or "careless" or "clumsy" anyway -- be more creative! be more accurate! Just take slurs out of your vocabulary completely. There is no acceptable way to use a slur.*
I can't be friends with people who use slurs. When I say I can't deal with this, I don't mean I can't deal with people who call names. That's a problem too, but it is WAY less common. I mean, every time you use these words in ANY WAY AT ALL it stabs me in the soul and there is only so much of that that I can take.
[this is also inversely true.]
This is also inversely true (calling people 'fit' when you mean you find them attractive is fatphobic and saying 'be a man' when you mean 'be brave' is misogynistic, etc) but that's a next-level analysis and at least it doesn't involve slurs. I don't like it but I can tolerate it. it's more like you gave me a hard finger-jab in the soul rather than stabbing.
*except for reclaiming: that is, society puts you in a group labeled with a slur, and you self-identify with that as a way of rejecting the stigma. For instance, I have reclaimed the word "fat" and use it as a self-label. I do not ever use it to mean something negative.