What Causes Difficulty In Having Penetrative Sex?

His penis won't fit in, please help.
I am a virgin, with a virgin boyfriend, and we both recently decided that we are ready to have sex. However, when we tried, his penis simply would not fit inside me. I am not nervous of sex. He is seven inches and quite wide and I worry that my vagina is too small to accomodate. Can anyone help us get around this problem please?