Oh, for God's sake. Someone's feeling cynical.
This is not how humans are portrayed in the SC2 universe, anyway. The human society of the future is much more peaceful and rational than the nations of today, and even at our worst we aren't as bad as many of the fictional races portrayed in the game. It shows as much lack of imagination to claim that human beings are the worst possible race in the universe as to claim they are the best.
And why does everyone pick on the last 2000 years? Human beings have been fighting for much longer than that. Is this some misguided attempt to blame Christianity in particular for the bad things of the world, or something?
It is not conclusively proven that Humans and Syreen are cross-fertile. The only things known for sure are that Zelnick is telling is telling the tale to his "grandchildren" at the end of the game, and that those grandchildren are not obvious Human/Syreen hybrids. There are several explanations for this:
- Humans and Syreen are cross-fertile; human pigmentation genes (melanin) are dominant and thus will not appear in offspring if a Syreen appears at only one point. Syreen are, for all practical purposes, another race of humans (but are not Humans -- this term reserved for the Earthlings).
- Humans and Syreen can breed, but the offspring are not fertile; these children look human because they are -- Zelnick and Talana's children adopted. Syreen are hominids, but not (small-h) human.
- Humans and Syreen cannot breed; Zelnick and Talana themselves adopted. Solves all these problems neatly.
- Humans and Syreen may or may not be able to breed. We don't know, because on Unzervalt, "Grandfather" is simply a term of respect for the aged. Unlikely at best.
Er, we don't know exactly to what degree Syreen are blue, or why. I'm assuming there's some blue pigmentation in their skin, but whatever it is is actually pretty faint, and may only appear as blue as it does because of the radiation of their native sun; in the scene where Talana appears to Zelnick at the end of the game, she barely looks blue at all, probably 'cause it doesn't show well in the light of the Starbase. Maybe the light on Unzervalt is more similar to the lighting on the Starbase than to the Syreen ships' lighting or Syra's lighting.
See the image here for what I mean: 
I see little reason to believe humans and Syreen aren't meant to be implied to be cross-fertile. Maybe it's me reading into the authors' minds, but then the authors did choose what to show and what not to show intentionally -- we shouldn't treat the game as though it were a real-life record, random bits and pieces of information falling into our laps; it's like a real-life documentary, where the director shows us certain facts because we're meant to take home a correct interpretation based on those facts. In this case the final choice is almost certainly invalid -- kids might address some random elder as "Grandfather" out of respect, but they wouldn't refer to the man's wife as "Grandma" in the third person. Believing the children are second-generation adoptees is even weirder (human/Syreen offspring being infertile, like mules, is even more of a wrinkle than human/Syreen offspring being cross-fertile at all).
I think if we weren't meant to understand that humans and Syreen are cross-fertile, we wouldn't have been presented with these kids as Zelnick and Talana's grandchildren in the first place, and they would have just been adoring kids asking Zelnick the war hero about his past. We are meant to take home a pretty big shock at seeing that Zelnick and Talana are married, and that they've reproduced.
Also I really dispute that there's any need for the kids to be "obvious hybrids", and that if they aren't it casts doubt on what would otherwise be an obvious implication (i.e. if Zelnick and Talana were both humans you would *never* think the ancestry of the kids at the end was in doubt). Grandchildren of an interracial marriage often don't clearly show signs of being interracial and gravitate to one side or the other of the racial spectrum by sheer genetic randomness, whether a particular gene is dominant or not. Especially given that everything about the end sequence is meant to emphasize the sameness of humans and Syreen rather than their difference -- Talana is shown to have very human-looking skin with only a hint of blueness, she speaks in a very casual, human tone without affectation, and so on. I don't think we're meant to think that Syreen are anything more than another Homo sapiens subspecies -- and indeed, the physiology of Syreen and humans are so darn similar that it'd be just as surprising in its own way if they were so physically alike yet were genetically different enough to not be cross-fertile (all other hominid subspecies look different enough from us that it's somewhat unlikely we'd find an individual of them sexually attractive, even with our closest neighbors, the Neanderthals).