This is the one term of Catholic Church doctrine that is misinterpreted by so many people. Miss-used might be a better way to say it. I get really tired when I hear people makings jokes about the Immaculate Conception, when they are really trying to reference the Virgin Birth. I don't mind you making fun of my faith tradition, I just wish you would bother to take the time to get to know a little about it before you go spouting off your big mouth.
This quote below is a wonderful explanation as to why Mary had to be conceived without sin in order to be prepared to be the Theotokos (God bearer). God makes sense if you just take the time to think logically about how this was all set about.
Anyway, Happy Feast of the Immaculate Conception.
Had Infinite Purity chosen any other port of entrance into humanity but that of human purity,
it would have created a tremendous difficulty, namely,
how could He be sinless if He was born of sin-laden humanity?
- Archbishop Fulton J. Sheen
(On the Immaculate Conception of the Blessed Virgin Mary)