Right cells of a single individual co-operate, why should sperm-cells be any different.
Does this mean monogamy is unnatural? Or at least at one point was not the norm? And still isn't? As why continue to optimize for co-operation?
I wouldn't call monogamy unnatural. But it may be a more recent invention for humans. We still aren't sure how long it's been the norm.
It's possible that humans used to rape more often. It's also possible that humans practiced forms of polygamy. Either way, cooperative sperm makes sense. It's one of those things where a simple mutation would provide a clear benefit over the competition.
You could expect that if each of them were an individual. Are they? One could say that all sperm-cells in a single ejaculation are part of the same single man.
Therefore the principle still holds, but it could be re-phrased as:
"Every sperm-cell for the donor-man"
Each sperm has a different DNA, they are like siblings. It's expected some cooperatio, but also some treason and backstabing.
[1] Ignoring some interesting cases.
That wouldn't be surprising, but I don't know if that is true.
I wouldn't not be surprised either way. A species survives not because its individuals do but because its genes do. Taking all available genes and spreading a subset of them amongst sperm-cells randomly is an interesting strategy.
It might be even optimal because the traits of a sperm-cell that make it survive during conception are not the same traits that help an individual thrive outside the womb.
If the 'fastest swimming' sperm-cell would always win, it would mean that in future generations faster and faster sperm-cells would win, but that would not necessarily help the survival of the species.