You're also free to join the anti-Belgium society.....
:)
As I remember it (founding member here), the goal of the Mars Society is to further the goal of human exploration of Mars. If it's your reasoned opinion that this is best accomplished by first going to the moon then your voice is no-doubt welcomed.
And I for one can attest to your passion and dedication to space exploration Raoul but if the focus of TMS isn't shared by you then theres always the Mars Institute, The Moon Society or some other faction within a fraction.