I have never seen such adverts on this site, although if they did exist then yes I would be highly disappointed since this is essentially an academically oriented site and is targeted (in part) towards minors.
However, I would say that the problem might actually be with your computer and not the site. If someone using that computer has been visiting some sleazy websites or has clicked on a link in a spam e-mail then quite often you can have a ‘virus,’ spyware or adware that would hijack normal adverts and replace them with the things you’ve described. So first I’d check your computer and find out what others who have access to it have been up too…
Also, it could be an issue with the advert hosting site if CC outsources that to a third party…