Everyone says, they believe so. While the Google Analytics terms of service does not specifically reject sites by theme, including not rejecting adult sites - they do say that the service is free for those with AdWords accounts.
In section two;
The Service is provided without charge to You for up to 5 million pageviews per month per account, and if You have an active Adwords campaign in good standing, the Service is provided without charge to You without a pageview limitation.
So if your adult theme site gets over 5 million pageviews, then technically, you would need an AdWords account in good standing. Since Google AdWords has a content policy that disallows "Sexual Content (Adult)" then those adult sites would technically not be accepted to use Google Analytics for free.
Forum discussion at DigitalPoint Forums.