Eugene Cho at God's Politics writes:
"I personally don’t care what you eat, drink, hunt, or watch as long as it isn’t porn."
This line stuck out to me because I think you could argue that "what you eat" or "hunt" is at least as fraught with necessary moral consideration as watching pornography. The connection is pleasure. People watch pornography for their pleasure (and if you consider there to be immorality either in its production or its viewing, you'll consider that pleasure problematic). In the modern developed world, people eat meat (or hunt) for pleasure. There is no other compelling reason. It is not necessary for survival or health. It is tied up with tradition and socializing, but that in itself would not be justification for otherwise immoral activities. But to enjoy this pleasure, an animal is required to suffer and die. If pornography is to be worthy of moral condemnation, I think that partaking in the suffering and death of an animal for one's own pleasure is at least worth moral consideration.
On a C-Span2 Book TV discussion, Jonathan Safran Foer tried to develop this point. When Frank Bruni raised the point about eating for pleasure (seeming to defend meat-eating on the grounds of pleasure), Foer responded by asking why the pleasure of taste seemed to trump morality in ways our other senses do not. While sex is pleasurable, humans place moral limitations on its enjoyment, and we wouldn't allow people to slaughter animals if it pleasured their sense of sight or hearing (as we wouldn't allow a person to rape an animal for pleasure). Unfortunately, Bruni never responded to the issue Foer raised (he used that moment to take offense that Foer uses language for animals that we typically reserve for humans). But I think the connection is worth considering.