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"