I've seen it theorized that human/qunari pairings always result in qunari similar to how human/elf pairings always result in humans. Far as I can tell, it's baseless speculation, but it's an interesting possibility.
Though apparently within the qun, they don't allow any crossbreeding between species, which I found interesting. The point of the Qun's selective breeding programs is to constantly create superior individuals, not racially pure individuals.
The fact that there's a ban on it speaks to me that they can interbreed, but that the results are found...lacking by the Ben Hassrath.
I find it interesting because before finding this out, I would have thought their view would have been more similar to that of the Horde in warcraft lore(before that lore got butchered), where the orcs would intentionally crossbreed with humans and ogres to produce more cunning or stronger individuals, respectively.
Hmm... Could it be that a qunari-human hyrbid may be infertile? You know ligers? Ligers are a hybrid cross between male lions and female tigers. Their offspring is often infertile (Not always though. I guess it depends on the individual?).
A qunari-human hybrid may be physically weaker than "normal" qunari (Though maybe still stronger than a human, idk). If they are infertile on top of that, it would be seen as a waste of recource, because normal qunari tend to be more useful to society because they are more resistent.
So all in all you would get a weaker, less resistent qunari, who wouldn't even be able to fullfill their role for reproduction.
Just a thought.
I am wishing for a history and biology lesson from Iron Bull about qunari 
He's still kind of a follower of the Qun, so I am really curious if he will try justify some of it's rules or explain them to us. I hope the general view on sexuality in Qunari society will be brought up. He could tell us the reasons why interspecies-relationships aren't allowed and tell us his opinions on the whole selection-breeding and forbidden love-making stuff. I want to push the big boy into some direction in regards to the Qun.