You already have to show ID to see rated-R movies in the US. I don't see how this is any different.
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[edit] and that doesn't just mean “okay jimbob is a dirty dirty boy.” It’s also a handy way to create a registry of whatever the handlers think is the target perversion du jour.
[edit][edit] … and it's not even the government who's keeping that database, it's pornographers. Regardless of your political leanings or trust in the gov't, can you imagine a less trustworthy party to hand off your ID to? mein gott
But since they've moved most ticket purchases online it's very likely they do maintain such a database now, and monetize an "anonymized" version of the data.