Our objection to the "ignorance of the law” defense is based upon the principle that subjectivity tends to water down the moral authority of the law. To say that the illegality of murder or rape hinges on whether we are aware of its illegality is to invite relativism. The law of gravity is not contingent upon our perceptions. We did not remain suspended in the air before we learned about gravity in grammar school. We were bound all by the law of gravity even before we were aware of its existence. Similarly, we are bound to moral laws regardless of our perceptions of them.
The more obvious objection to the "ignorance of the law" defense is that it would invite fraud. People who murder and rape will also feign ignorance in order to escape liability. This requires no further elaboration.
To tolerate subjectivity in defining life and its onset would cheapen life itself. It would also invite the murder of teens at the hands of those who assert that life begins in the twenties. Tolerance is not always admirable. In fact, it is often unworkable.
Scott Klusendorf sums the issue up nicely noting that "The science of embryology establishes that from the earliest stages of development, each of us is a distinct, living, and whole human being. True, we have yet to mature, but the kind of thing we are is clear. This is settled science. Of course, there remains a philosophical debate on how we should value humans in their earliest stages of development, but let us not confuse the value question with the empirical."
Most people know when life begins. The issue is not the value we place on the views of dissenters. The issue has always has been the value we place on life itself.