It seems that "fungibility" needs to be treated as a continuum.

Agreed.

In what way are sperm fungible?

It mostly seems that way because they're massively overproduced, but you are right to question that.

I think I'm going to turn to my claim about future development as important in identifying sperm as more fungible and fertilized eggs and beyond as less fungible, but I agree that claim is weaker than I thought it was when I made it.