How? NoScript (which I use) stops random scripts from running. To watch flash videos (of any kind) one must allow the script to run. Once you've done this NoScript isn't providing any protection anymore.
There's usually all kinds of Flash and scripts on porn sites, esp 3rd party. These pose more risk than the main video of a reputable porn site that seeks to keep its viewers. So, as usual, you use NoScript to limit your risk by trying to block third-party stuff where possible. You can also sandbox if you want.