I don't happen to share this view, as I have explained before. But it does make a certain amount of sense.
With all due respect, it doesn't make sense to me. It seems to me the equivalent of saying that because most people are going to die from natural causes anyway then it is ethically acceptable to deliberately target and kill some of them. Implantation as a dividing line between when it is acceptable to kill and/or experiment on a human being-in-fact is completely arbitrary because a human being's location or access to nourishment has nothing to do with the intrinsic dignity and worth of that human being.
Cordially,
But, of course, absent implantation, the blastocyst will never become a human being, and will be discarded by the body. But for implantation, a blastocyst is not, and cannot be, a human being. It's "intrinsic dignity" consists of two options -- implantation and commencement of the process of fetal development (of becoming a human being, in other words), or termination by its host.
If the threat of "arbitrary" line drawing is paramount, then of course your proposed line of non-protection for gametes, but protection for fused gametes (presumably meaning that the moment of fertilization demarks the line of non-acceptable artificial destruction), must be eliminated as well. As I noted above, your attempt to delineate a gamete as merely a "part" of a human and therefore free from protection fails the simplest of tests - can a human exist without this ostensible "part"? Indeed, if your reasoning is accepted, then the blastocyst itself is a mere "part" of a human being unentitled to protection, since an unimplanted blastocyst cannot alone become human.
I am not making light of this, by the way. I fully understand the difficulty of drawing lines. But since we choose to draw lines for many reasons, I tend to think that a closer approximation to a "but for" line is the more appropriate measure.