According to the Qur'an, Christians commit shirk because we believe in the Trinity. Yet it is clear that Muhammad (and, apparently, Allah) had no clue what Christians actually believe. According to the Qur'an, Christians believe in a Trinity made up of God, Jesus, and Mary, and they're three separate gods (5:116-117)! (Note: If the Qur'an were the Word of God, we would expect it to accurately represent what we believe.)
No Christians believe that Mary is part of the Trinity. Moreover, belief in the Trinity is thoroughly monotheistic. Hence, the question arises: When our belief is represented accurately, does it necessiate shirk? Of course not. Put simply, if God is a Trinity (which He is), then believing in the Trinity is not shirk, for no partners are being associated with the one True God.