A DigitalPoint Forums thread asks if Google will allow you to track your adult site using Google Analytics, Google's web analytics software.
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.