I learned a lot from the comments. Mockery of Mormonism is widely agreed to be hilarious because it is "obviously" absurd. For example, it is "obvious" that Joseph Smith is a con artist. When a similar treatment is aimed at mainstream Christianity, exposing the "obvious" lies, it seems tacky and offensive because these lies are widely believed.
That said, I can't tell if the original post was designed to troll people who would be offended by its style, or designed to amuse people who already agree with its message.
Today's post, The Amazing Virgin Pregnancy was originally published on 24 December 2007. A summary (taken from the LW wiki):
Discuss the post here (rather than in the comments to the original post).
This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was Zen and the Art of Rationality, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.