These are not difficult topics to argue against, as discrimination in any form of business, housing or banking is illegal and has been for quite some time. So my first argument would be that it is often against the law.
My second argument would be that the majority of society does not approve of discrimination based on race or gender, and the words racist and sexist are considered serious accusations, both legally and otherwise.
My third argument would be the affect it has on both parties. Obviously to be discriminated against is dehumanizing, discouraging and frustrating. It denies someone their humanity, but it also affects those who discriminate, as it traps them in a permanent cycle of hate or superiority/inferiority which isolates them from large segments of the population. This hurts them too.
Since you're only calling for arguments against, there isn't a lot to compare or contrast.
Three arguments are:
Rights and Justice. One can compare and contrast between these to determine which is the strongest or weakest.