there's a lot of really decent paysites that have mobile version of their sites. some better than others. that is, if you feel like not ripping off porn. you really should do your homework and consider it.
Why don't you just download the movies you want and convert them to mp4 video? That's the format that plays back on the iPhone.
It's simple and free. No need to pay for anything. I've done it with several movies on my phone. Granted they weren't porn, but the method still applies.