The Ubuntu Code of Conduct is something that's designed to ensure two things. Firstly, and perhaps most obviously, that discussions don't end up bogged down in personal abuse with no useful conclusions being drawn. Secondly, and perhaps more subtly, to ensure that potential contributors aren't put off by an atmosphere of perceived hostility.
The second point has been most commonly raised with respect to women in computing (perhaps most notably by the FLOSSPOLS findings), but it's not limited to that. One of the reasons I'm not involved with Debian nowadays is the atmosphere on the mailing lists. If you're about to make something that you expect to be a controversial point, realising that the likely result is going to be a certain number of responses implying that holding that opinion is stupid is a pretty strong incentive to avoid posting in the first place. Suppressing the expression of thoughts isn't beneficial, even if you're not doing so deliberately.
Now, the obvious response to this is to accuse the CoC of enforcing some sort of Tellytubby land where people are nice to each other and it's all a mutual love-in. Which isn't, in fact, the case. Disagreeing with people is fine, but disagreeing in a factual and precise manner is significantly more useful than disagreeing in a manner that gives the strong impression that it would have been more helpful if the original poster had died rather than sending the mail in the first place. You're not even obliged to behave in a friendly way towards people - just avoid behaving in a clearly unfriendly way. A good rule of thumb is probably that if you think it would potentially result in a complaint to your boss if you did it in a moderately large company, then doing it on an Ubuntu mailing list is a bad plan.
So, the original point. The joke itself implies that there's no way to know why women behave the way they do, even to the point where a (presumably) omniscient god has no idea. Unsurprisingly, telling a large body of potential contributors that their behaviour is irrational isn't a great way making them feel like their contributions are welcomed. But, oh no! It's actually ok - we're supposed to replace "women" with "newbs", because suggesting that basically your entire pool of potential contributors that their behaviour is irrational is much less bad.
We then get to Caroline's post, an expression of frustration about an aspect of life. It contains the word "Fuck", which doesn't seem unreasonable in context. There's then a complaint from someone claiming that it's offensive, which means we have to consider a few things. Firstly, is the author of the complaint actually offended, or just attempting to claim that there's hypocrisy involved? Given the context, I'd suspect the latter. Arguably this isn't really the point, though - the issue isn't whether a given person is offended, it's whether it'll offend someone who would otherwise potentially make a valuable member of the community. Right now, I'd suspect that we (as a community) don't really object to it strongly. Google suggests that swearing isn't unheard of on the lists. While swearing in the context of a personal attack is pretty clearly a violation of the CoC, people don't seem to be objecting to it in general.
So, is it unacceptable to swear within the Ubuntu community? Right now, I'd say that (in appropriate contexts) it's not a problem. If a moderate number of people claim that the current degree of swearing dissuades them from being involved, then it may become a problem. Accusing someone of hypocrisy for swearing (not clearly a CoC issue, especially in context) after they claimed that a sexist joke might be, well, offensive? Yeah, I'd say that that one's pretty clearly unacceptable. Telling people that it's pretty funny and that they're wrong to be offended? History suggests that it's not a good plan. If you think someone's wrong to be offended, then a good first step is trying to figure out why rather than just dismissing them.
Of course, there's also the entirely secondary issue of what the roles of planet sites in the community are. Personally, I like to think of them as opportunities to see what people are really like rather than being reflections of people's behaviours on project mailing lists. From that point of view, I think that the bounds of acceptable behaviour should be a little more liberal. If people disagree, that's not a problem and things can change - but expecting people to change their behaviour before there's any sort of general opinion that it's unacceptable isn't ok.
Which leaves open the question of whether posting sexist jokes to mailing lists is on the acceptable side of the CoC or not. I'd be surprised if the ubuntu-uk community genuinely thought so, but if they do then there are pretty clearly issues that we need to solve. As far as I'm concerned, justifying that sort of behaviour has no place in Ubuntu.
 Yes, I'm aware that there's a certain degree of irony there
 For the record, yes, I do believe that women often behave in irrational ways. So do men.
 I find it surprisingly hard not to write that as COCK. Too much Jerkcity
 Which is, to an extent, why this journal isn't on Planet Ubuntu - there'd be potential for things I said to be mistaken for some sort of official Ubuntu position, which would end up limiting what I could say, which would kind of ruin the point
 Must... resist... temptation...