I personally agree with you whole heartedly, Tony. You're merely upholding the companies best interests and if some employees find that a problem, they'll just have to cope. How many smokers at your work get inconvenienced by non-smoking policies? Did the company make special concessions for them?
Genixia's comment: I guess that you perhaps need to make this clear - That you really don't care what they look at within the bounds of what the company policy allows them to view. is spot on. the company has control of the data lines and you're being proactive in upholding various policies. The fact is that adult sites are the most notorious advertisers and 'abusers' of netiquette.
If users want to look up those kind of sites, they should do so on their own equiment.
What's the companies policy on personal phone calls?