WEll if you burn it kinda defeats the purpose of watching it on the pc by that point o_o just have a double copy for w/e reason. XD For me it depends; if I really REALLY like a movie I put it on a DVD and watch it on my TV so I can surf the internet while glancing over when I feel like it and watching the movie...
I rarely ever just sit straight through a film; watching for me includes blogging, researching, drawing, chatting and randomly surfing photo websites.
If I find something online that I *really* like, I'll save it to my hard drive, but I've never burned any of it to a DVD. We have a fairly decent collection that we keep on our (networked) media center machine. If I'm by myself, I'll watch it on my laptop, if I'm with the hubby, I'll watch it on the TV.
EDIT: I just answered this with porn in mind, but after re-reading the question, I'm not sure if you were asking about porn exclusively or all movies!
If you're asking about ALL movies, when it comes to non-porn, we still don't burn anything to DVD, and those are watched exclusively on the TV.