A simplistic comparison; if you want common sense, first consider this: illicit drugs kill people, but abortions do not because fetuses are not people. 
At most, they are _potential_ people, with that potential contributed by the woman carrying the fetus, who is already an actual person, and whose ability to direct her actual, already extant life is vastly more important than the _potential_ (i.e. not-yet-realized) life of the _potential_ person she is carrying.
Of course, the emotional reality is more complex than that, but the _ethical_ reality is that the ability of adult women to direct their own lives is vastly more valuable than the forced preservation of every single fetus that is conceived.
(There are already over seven billion of us, you know. We should do what we can to ensure that whoever else is brought into the world is wanted, supported, and properly cared for. Abortion is a difficult but necessary part of that process.)