If you are going to argue this, you will need to argue that it is unethical because of its effect on the environment. You can argue this in two ways. First, you can argue that the fluids that are used for embalming bodies are bad for the environment. When the bodies decay, the chemicals come out and get into the earth and later into ground water. Second, you can argue that embalming leads to burying (rather than to cremation). You can argue that burying bodies creates a huge demand for land that could be better used for other things.
If you argue it in these ways, you can say that embalming and burying bodies is unethical because it causes unnecessary harm to the environment.