I guess it depends on what kind of sponge, but I think in all likelihood since most sponges have no symmetry that this comes down to the same politics as an agender person choosing a bathroom.
Edit: let me preemptively be a pedant to myself and say that "sub-...-species" is wrong because "bilaterally symmetrical animals" is not a species. Flounder is itself a species AFAIK, not a sub-species of anything. It is a descendant of the common ancestor of all bilaterally symmetrical animals. There, now surely no one will find anything to be pedantic about :D
I appreciate that information. However, flounders themselves are not bilaterally symmetrical. I have caught many dozens of them and it's pretty easy to tell that they are not.
What if you take off the costume? Humans aren't entirely bilaterally symmetrical (at least not on the inside) and obviously not radially symmetrical so the paradox continues.