Testing the "future like ours" Claim

My objection begins, appropriately enough, with sperm and egg.  Marquis's central argument is that what differentiates a fertilized egg from an individual gamete is the "future like ours" that the fertilized egg could have.  He asserts that this is the fundamental principle that can make killing immoral even if no other reason is present.  He admits, however, that his argument "stands or falls on the moral status of the fetus."  It does not necessarily need to be a full human, but needs to have a future like ours.

Some people would assert that a sperm or an egg also have a possible future, to which the response would probably be that the odds of this happening for an individual gamete are so ridiculously slim they should not be considered.  Marquis, for example, says that contraception is no problem because there is simply "no obligation to maximize," implying that before the two are combined, it is not worth considering that they have rights.

I would like to propose a scenario which could challenge this assumption, in the the style of the “kill one to save 5 – organ harvest vs. oncoming train” dilemma.  We first ask 2 questions:

Should we protect sperm and eggs (or feel bad about the loss of sperm or eggs)?  Most people say no.  Should we protect a fertilized egg?  Significantly more people will say yes (understandably).

The third scenario, which makes the distinction more difficult, is this:

There is a stage during fertilization, before implantation, where the sperm has merely become attached to the egg, and the egg has sealed off its outer layer to other sperm.  No genetic material has yet been combined.  They are still two separate cells which are merely stuck together.  In Marquis's argument, however, we must protect them both because they now have the possibility of a "future like ours," but are not even really a single being yet. To me, this level of moral status of a fetus is not enough to grant the critical support his argument "stands or falls" on, and the argument, though generally very well thought out, falls.