The fact is that YOU will never know whether a woman prevented fertilization OR implantation. She probably will never know either. I think this is taking the argument to such extremes that it becomes....well, ridiculous.
Please consider the fallacy of your argument. You state that the fact IS I will never know whether a woman prevented fertilization or implantation. Well then, neither will you! The mere "fact" that it's unknown should give anyone that is pro-life more than a little concern. Are you seriously contending that just because a woman isn't aware if she prevents a fertilized egg from implantation that makes this okay?
"I think this is taking the argument to such extremes that it becomes....well, ridiculous."
I agree. It is a typical knee jerk reaction. Some say it prevents implantation, others say it prevents fertilization. Huge difference.
Knowing the facts would be helpful. As a nurse, this is how I have undertsood the drug works; by preventing fertilization in the first place. Not that different than spermacides, etc. Of course some people are opposed to all birth control measures so to them ,then, well...it would be irrelevant.