You sound like you've made up your mind that you're going to do it anyway, but I agree with the majority and I say don't. It'd be a really stupid way to get let go. Like others have said, get a cheap laptop or a tablet and use that instead. But using company property to watch porn is stupid. I know you want to make the case of "it's not on company time or property," but, again, why take the chance that it will come back to bite you?
Personally I'm not going to take any chances. I won't watch porn at work. Period. At my last job, there were quite a few people who were let go for porn on their computers or pics sent through the internal mail system (Really? How dumb can you get?). I wasn't ever stupid enough to do shit like that though. While it may vary from company to company, the monitoring programs are a lot more sophisticated these days and it's really just a dumb risk to take.