Here are some excuses:
A) because while Bike Smut does create a fair amount of content, most of the works are from other filmmakers, many of whom are interested in protecting their identity. No DVDs (not even to theaters wanting to preview it) mean that there is almost no chance that their content will end up online.
B) being in a space with others watching adult-themed material (be it violent, sexual or otherwise) creates a feeling vastly different from being at home. And it is not just because you feel more comfortable masturbating in front of your computer! (public/group masturbation can be awkward but is often exciting. Build up courage by trying it in front of a mirror, or your pets).
C) the value of bringing together of people who feel mature enough to sit though something and then have the stability of mind to discuss a topic afterward is difficult to overestimate. In a society where everyone is anonymous how does anyone establish what is obscene?
Y) We are lazy.