Who cares if it's normal? If you both enjoy it, go ahead and do whatever works for you.

As for stretching out your vagina - it was designed to be able to stretch enough to pass a baby's head, shoulders, and pelvis during childbirth. Even if your boyfriend has real sausages for fingers, they're nothing compared to childbirth. Loose vaginas are usually due to extreme stretching and muscle tearing during childbirth and/or poor muscle tone. Doing Kegel exercises (look them up) can help a lot to build up muscle tone.