Can I use Coppermine for a site where people can upload sex/pornphotos?
Yes. You can use Coppermine for any purpose you wish. The GPL grants you this right.
You're even allowed to ask for support if you're using coppermine to display porn and post a link to your gallery if that is needed for support purposes. We just ask you to post a warning with that link that it's not worksafe to visit that link (refer to board rules (http://forum.coppermine-gallery.net/index.php/topic,55415.0.html)). The license Coppermine comes with can be reviewed by reading the docs (http://coppermine-gallery.net/demo/cpg14x/docs/index.htm#copyright). You'll find reference to the GNU GPL there as well.