In response to the claim that contraception "deprives both the sperm and the ovum separately of a valuable future like ours", Marquis writes:
On this alternative, too many futures are lost. Contraception was supposed to be wrong, because it deprived us of one future of value, not two.
This is too quick. We can imagine "fusion" cases where two people (A and B) merge together to share a single future C. (It helps if A and B are intrinsic duplicates, or at least extremely similar.) As in fission cases, we might not want to describe this as a case of strict "identity" across time, but it's clearly compatible with what matters in survival. That is, even if the logic of identity prevents us from saying both that C is A and that C is B (because A isn't B, and identity is transitive), we should still hold that C's life constitutes a "future of value" for each of A and B, such that it'd be harmful to each to deprive them of this future by killing them before they merge.
Since it's possible to wrongly deprive two people of a single shared future, Marquis' response here is insufficient: more work is required to explain why contraception doesn't deprive both the sperm and egg of a valuable future. (I think there are two respectable answers here. Pro-lifers should appeal to a kind of biological essentialism, and say that gametes are just a fundamentally different kind of entity from human beings, and hence do not develop into the conceptus, but rather are replaced by it. But my own view is that only persons have futures in the relevant sense, so I don't think fetuses are harmed by death any more than sperm are.)
Marquis continues (pp.201-2):
One might attempt to avoid this problem by holding that contraception deprives the combination of sperm and ovum of a valuable future like ours. But here the definite article misleads. At the time of conception, there are hundreds of possible combinations... there is no nonarbitrarily identifiable subject of the loss in the case of contraception.
The notion that victims need to be "identifiable" yields atrocious results. Suppose a large roomful of people are about to be gassed. A teleportation machine is set to randomly choose one of these people and teleport them to safety. Would Marquis have us believe that it'd be okay to turn off the machine and let everyone die, just because "there are hundreds of possible" beneficiaries, and no particular person we can pick out as the one who would have been rescued?
Again, we need a different explanation of why it is that mereological sums of sperms and eggs aren't harmed by contraception. In particular, we should say that these gerrymandered "objects" aren't the right kind of entity to be the subjects of future experiences. (Only persons, or organisms, or some such, exhibit the requisite internal unity.)