Through a deep lack of cultural understanding that Muslims seem unable and unwilling to remedy, it would seem that they think that Christians believe in more than one God.
I don't think it's culture or a lack of understanding that makes Muslim's see Christianity as polytheistic. It's what the Christians say that gives them that idea. What would "God the Father," "God the Son," and "God the Holy Spirit" be, if not polytheistic?
The Athanasian Creed states,
"
So the Father is God, the Son is God, the Holy Spirit is God;
And yet there are not three Gods, but one God."
It's as though they think they can wave some kind of magical wand and it will legitimize an otherwise nonsensical statement.
To be sure, there is a "God the Father" mentioned in the scriptures.
1 Cor 8:6,
But to us [there is but] one God, the Father, of whom [are] all things, and we in him; and one Lord Jesus Christ, by whom [are] all things, and we by him.
There is but one God, and that would be the Father of Jesus.
Whereas there is mention of "God the Father," there is nowhere in the scriptures that we find "God the Son" or "God the Holy Spirit" ever mentioned. But even if there were, Corinthians clearly states that God the Father is the only true God. To be more precise, I should say that Yahweh is the only true, almighty, creator of heaven and earth, God. The scriptures do mention other gods, including Jesus Christ, but it was completely understood by the ancient Near Easterners that the word "god" simply meant someone with power and authority. Jesus would certainly qualify in that regard since God granted him those things. That alone should end the debate; why would God have to grant Himself anything at all?