Most things aren't math equations. All distinctions in the real world are messy, seemingly arbitrary at some level. Can you define what it means for something to be alive or dead? I seriously doubt it. Yet we do not dispute the existence of such distinctions. The concept of life improves our understanding of the world despite its slipperyness.
IMO, moral truth exists as much as scientific truth we just interact with/discover it differently. The Stanford Encyclopedia of Philosophy is a good place to start if the topic interests you:
I agree based on lots of personal experience with and research on people who talked utilitarian but acted different when the law's enforcers were focused on them. I fought with myself over this in many situations balancing the struggle between my own survival/well-being vs supporting principles. Takes discipline and sacrifice to practice utilitarianism. What it took me led me to be suspicious of people who preach it who spontaneously do what benefits them esp at others' expense even in small ways.