I watch it alone. In fact, my girlfriend is uncomfortable with the type of porn I like and I'm uncomfortable with the type of porn she likes, so it's for the best.
For me it's both. I do enjoy watching it alone if I know I will have the time to give myself the release I know I will want when it's over. On the other hand I do find it a turn on in a big way to wstch it together.
We watch together and alone. Depends on who has the time. Porn is good at learning ideas for new ties or positions or any fun new exciting things we haven't tried before.