It's not exactly common in the US - certainly not where I live. I too, am the hinge of a vee.
For the first couple of months of dating my boyfriend, I spent 3 days a week at his place and then 4 days with my husband at my house. We have transitioned now to my boyfriend moving in with us. Only recently did we become out to all of our extended family - but I definitely thought it important that they knew, so we wouldn't have this dread and anxiousness hanging over us. It would just temper our happiness too much. If your boyfriend has said that he would have a difficult time choosing you over his mother - well, you know where you stand. I guess you need to decide whether you want to have that worry or not.
A modern day polyamorous polygamist My Online Journal
Me, center of a MFM V-shaped polycule (38F)
Wife to DarkKnight (40M) & PunkRockAwesomesauce (42)