You know, I got curious, so I started doing some google searches. The earliest post of the boyfriend-living-with-monster-girls comics I can find is 2009. (There's a manga series by him as well that started in 2012 and is getting made into an anime.)
Monster Girl Quest was released in 2011, so the Living With Monster Girl thing does actually predate Monster Girl Quest. I have a feeling though, just from stuff I've seen, that monster girls are a "thing" that predates both of them, and people have just approached it with greater or lesser levels of Penis Frenzy.