Was that the best example of sexism you could find, OP? If so, then I feel you're really over-sensitive because all sexist jokes aren't automatically offensive. That joke by Galaki clearly didn't make any ridicule of you as a thread maker. It was just an innocent joke.
One could argue though that his post somewhat contributed to diminish the weight and impact of your topic to a little degree, but that's got nothing to with the sexist nature of the joke, but it's a problem with all off-topic posts posted early in a thread. It irritates me a lot when I make a thread, putting lots of time and energy creating it and the first few posts are short one-liners that aren't specifically addressing anything I wrote. I would guess others feel the same. But that's an entirely different issue than sexism and other forms of discrimination, and it also can't be avoided as it's not a formal breaking of rules.
Also, there's such a thing as proportions. If this site actually was flooded by sexist jokes (it isn't), even just innocent ones, then it would/could be an issue, but something up to the moderator's general judgement on how to handle.