"thought someone may have experienced the same situation"
Sure, I experience this same situation very often when my wife or my son plays YouTube music video clips on their smartphones over a Bluetooth loudspeaker or on a PC all day with auto-play mode on in the YouTube app or on the YouTube website. Each music clip produces one or more such DNS lookups for a subdomain of googlevideo.com. And this is reflected in my OpenDNS domain stats, of course. So I exactly know what you're talking about!
The difference between us: I would not even have thought that this could be related in any way to porn. You seem to be hyper-sensitive believing every junk you "read that there were loopholes". There are indeed loopholes, but these are hardly published anywhere and are almost of a complete different nature than you think they are.
"I asked about not so obvious domains"
Again, nobody should publish such material here, no matter if domains or websites. Especially if they are not obvious, they will definitely become obvious then...
If you become aware of such domains, your best bet is to report them here:
https://community.opendns.com/domaintagging/submit/
