I discovered that my blog was infected with malware that inserted spam text into the version displayed to search engines. So, the blog looked fine when accessed directly, but showed a bunch of sexual enhancement product ads when viewed through Google’s cache. So in case you were wondering: no, that was not intentional! This is apparently a common form of attack, since it’s not obvious at first that something is wrong.
I believe the malware has now been removed and the blog updated to prevent a recurrence. It may take a bit of time for the Google cache to update and reflect this.