Even if sexual imagery, or anything else for that matter, contributes to the harming of someone, it doesn't follow that it is immoral, or that it shouldn't be done. Driving cars leads to thousands of deaths each year, but driving one isn't therefore immoral. Here's another example, having children is an enabling cause of child abuse, since if we didn't have children, there wouldn't be any such abuse... Instead of condemning a practice by pointing out that it is an enabling cause for harmful behviour, what should be looked at is the totality of benefits and harm, which isn't a simple thing to do.
To sum things up: Some people think that pornography is inherently bad. Others don't. Some people think that it can be Art. Others disagree.