Yes, but that would be spyware that is hijacking your browser and loading bookmarks into your bookmark folder favorites. Use Ad-Aware 6.0 SE to clean your computer of illegal or malicious entries and spyware. You can click "Custom Scan" to set it to check the Favorites and bookmarks for malicious sites. Get this program here: http://www.download.com/Ad-Aware-SE-Personal-Edition/3000-8022_4-10319876.html?tag=lst-0-1
Yeah it happens, but usually when one visits porn websites. That would mean that that person likely visited either a warez website or more likely a PORN website. I just hope that you are not a parent asking so you can find out what your son has been visiting.