Porn sites are notorious for embedded code
Historically, yes, but the trend has been creeping into non-porn sites as well. A lot "free" services out there now have this sort of stuff built into it - a lot of deceptive advertising trying to get you to download something or scare ads claiming your system is affected.
I see this mostly at file-hosting sites (trying to download mods for my son's Minecraft game is almost a game in itself, jumping through all of the hoops and avoiding the misleading "Download" links that will get you the adware instead of the file you're looking for) and some blogs.
The most annoying is the "pop under" MacKeeper ads, due to not only the deceptive nature of the ad, but the scummy practice of using pop-unders.