To be "half" something for us means that 12 of our 24 pairs of chromosomes has a specific origin. In order to contribute to our genetics like that both of our parents must have DNA, therefore they must be physical. Demons for one are not physical and two do not have DNA.
Just to add to Demyan's answer. Even if demons were physical beings (they're not this is just for those that might argue and say they are) it's assuming they also have 46 chromosomes that possess the same genes as humans or at least VERY similar, the odds of which given that it would entail at least 3000 years of isolated reproduction are pretty much non-existent, same reason why you can't cross a cat and a dog. If someone's told you they're a half demon and that they suspect you're one too then they're simply role playing, ignore it and move on or they'll draw you down the same path.
Very good point Yakov, even if they were physical beings they most likely wouldn't be homo-sapiens which means, not being of the same species, there would be the factor of gametic isolation. We wouldn't even be compatible parents.
Didn't we already have this thread before?
I'm sure we did this before when someone said Succubus (I'm sure I spelled that wrong and most likely spellcheck will make it some weird word or just fix it) take the look of a woman and collect semen then turn it demonic then become a man and put it into a woman to make a Succubus/Human (I think the person who made the thread thought they were a Succubus and human mix)
I'll respond to this thread like I did that one: HahaNO
Indeed. If they were able to produce offspring wouldn't there be a lot of partial demons by now? I mean, having sex with human women is pretty much all incubi do, so wouldn't that leave us with whole bunch of partial demon-ness after a few generations?