God could have done lots of things. Out of all history we do not know if there where other women who may have been good choices to be the Mother of the Second Person of the Holy Trinity. He could have made the world without gravity, or any gravitational pull by the moon effecting the tidal movements in the ocean. The fact is that He did those things. They are all real, and the fact is that the Immacultate Conception is a reality.
Most Orthodox, except for a few with a nasty bone to pick do not object to the concept of the Immaculate Conception, what they object to is the idea that the Pope, or any Council that does not include the Orthodox bishops of the world could or can pronounce any dogma. Many Orthodox priests and theologians I have spoken with believe that the Immaculate Conception is theologically sound. But that begs the question of why Dogmas are defined. The short answer is that there are times when long held beliefs in the Church that have not previously been defined dogmatically are questioned by groups or individuals. After the French Revolution there rose in Western Europe the errors of Rationalism, and later Modernism. Rationalist questioned anything that spoke of the supernatural, that included the existance of a soul and the existance of God. Our Lady has always been easy prey when it came to mocking the Church. As the long held beliefs of the Church where called into question, Pope Pius IX asked that the theological position and the history of that position be studied. Positions both pro and con where debated, and it was determined that the Church had held the Immaculate Conception to be true. The fact is the feast was first introduced in the East, (where Orthodoxy is strongest) and later migrated to the West was part of the discussion. In part to supress the Rationalist denyal of the Immaculate Conception, which many Catholics in Europe where being swayed by, (as at the time belief in the doctrine was not essential to remain a Catholic in good standing) it was determined that the dogma would be proclaimed.
After the proclomation of the dogma there where numerous attacks on it. The Orthodox attacked not the dogma but the idea that the Pope could unilaterally proclaim a dogma. Many Orthodox took the position that if the Pope said it was so, Orthodoxy must reject it, (throwing the baby out with the bath-water.) Some Anglicans called it the “Immaculate Deception” because of Anti-Papal feelings. Most mainline Protestants where simply Anti-Catholic and Anti-Papal so rejected the dogma or simply ignored it thinking that the Church simply reaffirmed that our Lord was concieved without the intervention of a human father.
If you have made it this far, then the Short answer is dogma is defined when there has been an attack (either directly or indirectly) on a belief that has been held by the Church.