Okay, if you really want porn on your iPhone, this is the best way to do it:
1. First, find and legally (wink,wink) download the movie that you want to use.
2. Next, you've got to reformat the file into a MPEG-4 Movie file. There are some programs that will do this process for you. For example, I use Vuze, and it can transcode movie files and make them compatible to play in iTunes.
3. After you've converted the files, add them to your iTunes library, and then you can sync them into your phone.
4. To play the movie, go into your iPod and you'll find it under videos. Enjoy!
The one I have on my phone right now is 239 MB. The quality is excellent, and when I tire of it, I can switch it out for a different one.