I do not see a contradiction in claiming that a) utility monsters do not exist and b) under utilitarianism, it is correct to kill an arbitrarily large number of nematodes to save one human.

The solution to this issue is to reject the idea of a continuous scale of "utility capability", under which nematodes can feel a tiny amount of utility, humans can feel a moderate amount, and some superhuman utility monster can feel a tremendous amount. Rather, we can (and, I believe, should) reduce it to two classes: agents and objects.

An agent, such as a human or a utility monster, is a creature which is sentient and judged by society to be worthy of moral consideration, including it in the social utility function. All agents are considered equal, with their individual utility units converted to some social standard. For example, Agent Alpha receives 100 Alpha-Utils from the average day, where Agent Beta receives 200 Beta-Utils from the average day. Both of these are converted into Society-Utils - let's say 10 Society-Utils - making an exchange rate of 10 Alpha:Society and 20 Beta:Society.

This is similar to how currency is exchanged. Assuming some reference point, perhaps an event which society deems is equally valuable for all agents (that is, society values it equally regardless of which agent experiences it), there exists a Utility Economy, in which there exists a comparative advantage; Agent Alpha and Agent Beta serve each other, producing more Society-Utils by trading than either could alone.

Left to the side are objects, such as nematodes. Objects are of value only to the extent to which they feature in an agent's utility function; for the purpose of ethical consideration, we consider objects to have no utility function. Therefore, it would be proper to kill nematodes to save humans - unless the side effects from killing so many nematodes began to threaten more humans than it would save. Similarly, animal protection laws would exist not because of any right of the animal, but rather the strong preferences of humans to avoid animal cruelty. This is consistent with the coexistence of factory farming and animal cruelty laws; humans don't much care about cows, but will fight to defend their pets (and creatures like them).

Of course, to some extent this is passing the buck to the "Utility Economy" to set fair rates, but I believe that a society could cobble together a reasonable exchange in which, for example, nobody's life would be valued trivially.