Currently there is an ongoing debate about a joke that was made during a lecture about free software. I have previously written about why I think it’s inappropriate with regard to children in the audience . For those who are interested in following this mess Matthew Garrett has written an interesting follow-up post with some useful links and a lively comment section .
I think that to some extent this is a symptom of a larger problem. That of speakers who take their audience for granted and don’t show them adequate respect. This is an easy trap to fall into, after giving many lectures which are well received it’s easy to become too egotistical and think of an audience as your right – rather than as a privilege that is earned by doing good technical work and explaining it in a clear and respectful manner.
Making jokes for a multi-national audience is difficult at the best of times, often jokes that work well in one culture will fall flat with an audience from a different cultural background. If you give a lecture that contains jokes then some of them won’t work, usually they merely fail by not getting any laughs but sometimes they cause offense. If you tell a joke in a lecture and no-one laughs then it’s probably a good idea to not follow up with any further jokes on that topic, if your speaking skills are not sufficient to allow you to make such a change to your talk in response to audience reaction then it’s best not to plan for a series of jokes. Regardless of the topic of the jokes it’s not a good situation if the majority of the audience is not amused.
Art is also subjectively interpreted in ways that vary according to the local culture and the definition of porn is even more subjective. In my post about appropriate talks about porn  the only situation I could imagine where showing a picture related to porn during a lecture about computer science was in regard to Lena and the history of computer graphics (the famous picture of Lena is cropped so that it is not pornographic), and that post did not receive a comment with any other suggestion. We could have a debate about where exactly the line should be drawn. But there are some situations where a line has been clearly crossed, such as a presentation about flash development which included a frontal view of a woman wearing semi-transparent underpants .
If you are going to give a lecture about art then there are valid reasons for showing pictures which may be considered to be porn by some people (I can’t imagine a lecture about Greek or Roman statues not having some serious nudity). But if the topic of your lecture is computer science then anything which significantly distracts the audience from that is a failure – even if it’s not offensive.
Presenting material that you find entertaining but which doesn’t interest the audience is self-indulgent. A small amount of self-indulgence will be accepted by the audience, but it needs to be short and forgettable.
If you respect the audience you have to respect feedback. For example if your wife or girlfriend thinks that your talk is great but women in the audience are offended then you need to take note of the feedback. If your presentation is designed to appeal to your friends and relatives then again you are making it all about you not about the audience.
Also when giving a public lecture you have to keep in mind the fact that even if you are famous in some field the majority of the audience won’t know much about you. The majority of the audience are not friends who have some background knowledge which helps them interpret your actions, and they aren’t people who have seen your previous lectures. Your lecture has to stand alone. Any defense of a talk which is badly received which involves a phrase such as “if you knew him better” or “if you had seen his other talks” is a weak defense. In almost all cases the audience should be expected to have no prior knowledge of the speaker.