Having travelled through SFO a few times, and each time been asked to go through a millimetre wave (or backscatter) scanner, and each time 'opted out' and gone for the search (in public), I'd like to know what is the proper reasons to give when they ask why I've opted out.
I tried the radiation argument the first time, just after they were introduced, but this argument doesn't seem to be too valid now since the millimetre wave scanners aren't very powerful.
I've tried privacy but not really known what to say afterwards other than the technical issues of caching, wiping images, etc.
Each one got some snarky comments from the security staff, and discussion from the one given the opportunity to feel me up, so I'd like to arm myself with a bit more information/argumentation for next time.