Animal hybrids are very difficult to concieve, yet alone survive gestation and birth is very rare for many. For example, Motty, the only confirmed African/Asian elephant born, died twelve days after birth. Unlike mules, African/Asian elephant hybrid actually have the same number of chromosome, which should have made them easier to breed.
They're different species. Humans and human mutants are much closer. Homo sapiens, and Homo sapiens mutensis, say. To bring up the earlier analogy - dogs and grey wolves may look different, but genetically they're almost identical and can interbreed without difficulty. Humans and human mutants will almost certainly be the same as there simply hasn't been enough time to build up major genetic incompatibilites.
I suspect the definition of "mutant" in the 31st century is more to do with physical abnormality than extreme genetic differences. It's possible for a lot of physical
variation to exist within a single species - look at dog breeds again - and they're not even regarded as a separate species from a grey wolf, just a race/subspecies, Canis lupus familiaris.
I want to repeat this again for clarity. Leela might not be infertile, but it does allow the writers to have a scientific (fictional) excuse for Fry and Leela not having children right away.
I don't believe she is. In anycase the sure the writers will make it happen or not as they choose. Whenever has real science ever gotten in the way of anything in Futurama? They bend the rules of science as they see fit. But yes, you're right they could.
Futurama needs to be different from The Simpsons if it ever reaches the point where the writers have to marry Fry and Leela. Futurama is an animated sci-fi comedy drama, not an animated family sitcom like The Simpsons, and it always should stay that way. I, as an animated sci-fi fan, do not want to have Leela impregnated before her wedding or two years afterward because that would be following too much after The Simpsons.
DXC was saying much the same thing in an interview somewhere or other. He wasn't ruling it out however...