[Note: I intend to use some mildly NSFW language in this post. I don't know why I'm so anal about warnings on that topic, but I am, so, well, shove it.] The title of this post is slightly misleading, but gets at the heart of what I want to say.
Thesis: I think there are circumstances in which it's valid, in an argument, to call your opponent an asshole. I think there are circumstances in an argument where the fact that your opponent is being an asshole is grounds for rejecting their argument.
The particular case I have in mind is when your opponent is arguing in bad faith, which I will define here as pursuing a purpose other than the reasonable and fair resolution of the argument.
You can identify when someone is doing it by the feeling you get during the conversation. If your opponent says something, and you feel stricken by an inability to refute it, because it's inconsistent with the basic presuppositions necessary to stage the conversation, they're being an asshole.
It's hard to respond, because what they say is usually something that would be perfectly valid in a certain kind of conversation, but which is, or should be, self-evidently invalid in the one you're having.
For example, if you're arguing with your boyfriend (let's assume you're monogamous, committed, and open to relationships with men) because he got head from a girl at a party the previous night, you might claim, "You cheated on me."
If he were to respond, "The construct of cheating in a relationship implies ownership, which is ethically indefensible," he's an asshole. Not because that's an indefensible ethical position, but because if he believed that, he should have brought it up at the start of the relationship.
It's not that he has to be wrong for his point to be invalid. It's that his point is inappropriate in scale to the conversation that's going on.
Another good word for what he's doing in this conversation is being pretentious. Ask me some time and I'll rant about that word for hours, but in this case it definitely applies.
On that count, I feel I should point out that it goes both ways. Throwing out abstract ethical arguments in a specific-case conversation about a real, immediate issue can be pretentious. It isn't always -- your hypothetical boyfriend had told you about his views on cheating early on, bringing up that you knew that was how he felt would be legitimate.
But it's also pretentious to attempt to pull an abstract conversation down to the level of immediate, every day life. This is something a lot of people who aren't liberal arts majors do. (There are definitely people who don't, and I love them for it. If you can tell the difference between a conversation you understand and one you don't, and subsequently contribute or not based on that knowledge, thank you. From the bottom of my heart.) People who do this are assholes.
For example, if you and your hypothetical boyfriend are having a conversation about the nature of truth, and he says, "Ultimately, there's no meaningful difference between truth and lie -- all language represents a distortion of reality in such a way as to make it convenient for other people to handle. It's impossible to make a true statement."
And you respond, "Bullshit -- it is, for example, true that you cheated on me last week." You're being a pretentious asshole. (We're assuming this is a different conversation entirely, not a continuation of the earlier one.)
In both examples, one of you is refusing to play ball by the implied rules of the context. In these cases, I think it's okay to just call the other person an asshole.
Why? Because implied rules of context are complicated. I could talk about them for hours, and probably wouldn't be able to get most of the people I talk to to understand what I meant. It's about as reasonable to expect someone to be able to explain exactly what someone's doing wrong when they break those rules as it is reasonable to expect the average man-or-woman on the street to explain the financial crisis.
People tend not to like thinking of their minds as being too complicated for them to explain. It's outside our comfort level in a way that accepting that our circulatory system is too complicated for us to explain just isn't. Assholes exploit that discomfort by cheating in ways most of us can't articulate, and don't really want to have to think about. Often they'll use the subsequent tongue-tied phenomenon to declare success. But that doesn't make them right. It makes them assholes.