These sorts of clever eco-friendly plastic alternatives get suggested every couple months or so, but nothing ever seems to come out of it. Virtually every single time there's either some serious defect that makes it impossible to commercialize or the plastic alternative is impossible to mass produce in a reasonable way.
In this case, it's both. The sperm-plastic is water-soluble, which rules out countless use-cases right away, and I can't imagine that there's any convenient way of mass-producing sperm in large enough quantities to compete with what's possible with oil. Not to mention, freeze-dried oily sperm paste sounds significantly less convenient for forming products than normal extruded or mould-injected plastics.