My wife's boyfriend has been living with us for over 2 years now. None of our family knows about the relationship, or heck, even that there's anyone else living with us. Since my family is only 2 hours away, I am not sure how the heck we have managed to avoid it, but we have, mostly by doing all the visiting and not asking them to come over.
Honestly I'm mortified of the day when my parents find out something is up. They're quite religious and won't understand at all. I have no desire to alienate my parents so I'm trying to keep it quiet.