This asks the question - what isn't at odds with human nature? Why do you think that utopian projects are necessarily at odds with human nature? Marx and Engels were sociologists, and in fact many modern sociologists are Marxists too. That doesn't mean they're right, but it's some indication that the "human nature" argument needs looking into.
And it has been looked into. Erich Fromm, a member of the Frankfurt School, wrote a book called Escape From Freedom in which he details the investigation of the human condition and attempts to identify what human nature is and to what extent it is malleable. The idea that "human nature" is suited to our current mode of production is as he notes a little fallacy stretching back to Freud's time.
The nature of humans is at least in part influenced by their environment. The mode of production we currently see (capitalism) is part of our environment. Are not changed men the product of changed circumstances? Perhaps Marx himself would agree with both you and I here:
"The materialist doctrine that men are products of circumstances and upbringing, and that, therefore, changed men are products of changed circumstances and changed upbringing, forgets that it is men who change circumstances and that the educator must himself be educated. Hence this doctrine is bound to divide society into two parts, one of which is superior to society. The coincidence of the changing of circumstances and of human activity or self-change [Selbstveränderung] can be conceived and rationally understood only as revolutionary practice." — Karl Marx, "Theses on Feuerbach, III"
To "human nature" I say firmly: human society.