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?
True, I will never know either. Obviously. You ask if it makes it o.k. because she may not be aware of whether she prevented a fertilized egg from being implanted? Yes.
I used contraceptions when I was younger, that prevented fertilization too. Is that bad? No. Is it any of your business or anyone elses? No. Who will judge? God.