This aspect is not true; you can play either male or female in ME1 and ME2. I know because having purchased ME3 recently I am replaying ME1 and ME2 so I can remember the story before ME3 and I am playing a different gender than I did the previous time (I can also say now that I am 20% into ME2 that ME1 is more fun with regards to game mechanics; even if ME2 is prettier but that is a different topic).
My main guess that only 18% of people played female on ME3 is because they played the 1st and 2nd one with a male character (since you did not have a choice there). And people get attached, or it would break character changing sex suddenly after playing male for 50 hours or so.
