Those who believe that the appearance wouldn't work out because how complicated genetics are, please review the codex again. I find it highly unlikely that this is an issue that gives any serious problem. To restate something for those who missed it, the hybrid child would not materialize until the end of the game. There would also be no pregnancy or anything like that. As for being a family person, it likely depends on your personal shepard. Based on the dialog I had when playing, it would seem that shepard would have no problem with creating a family after the business with the reapers is finished. Aris Ravenstar, I noticed that you included the term "unholy" in your argument. I don't know if this was meant to reflect your religious views or not but if it is I ask that you keep them off the forums.
As for the part about being accepted, I didn't say that everyone would except it, a lot of people probably wouldn't. Let's face it though, are people ever accepted by all for who they are? How many times in our history has a group of people thought themselves superior to others? How many times has a group of people been rejected by their society as "undesirable" and "untouchable"? How many times has the issue of race been used as a means to reject someone? In our past, interracial couples have been shunned by a society that just didn't accept them. These have been problems that have plagued our past and they in time have diminished somewhat. The key to ending this is to see people for who they are, to show that they are like us. Interracial marriage has been a way of helping this transition. The children of said marriages the bond formed to bring them together. You can try to argue that this is different, but it isn't. It's still the same concepts, just a slightly different scenario.
This sort of mission would have relevance to the game. The reapers, although they are the main enemy, are not the only main theme of the story. If this game were simply about shooting one enemy after another, it wouldn't be as good a game as it is. The name "Mass Effect" doesn't just refer to the mass effect fields, but also to shepard's impact in the story. His actions affect the mission, people's lives, and the universe as a whole. Many of his choices reflect his feelings, particularly towards race. For instance, save or abandon the council, save or abandon the collector station. His actions in these choices either bring the different people together, or seperate and tear relations apart. His choices reflect how he sees these people. In this way, the subject of a hybrid child ties into what is probably one of the biggest, if not the biggest, themes of mass effect.
Think about the mission itself for a moment. Shepard hears about a research station which has just developed genetic engineering that is capable of what others couldn't do before. The potential use of this research draws the attention of groups like Cerberus. Cerberus, as well as the other groups, would never pass a chance to take the technology, the potential is too great. With it they could alter people to give them a sense of superiority and develop subjects to be potent living weapons. Given access to the research, they could potentially gain a biological advantage over others. Regardless of what the research was originally meant for, either military or medical, the potential misuse and danger of the research would be reason enough for shepard to investigate. Why? Even if he is busy amassing an army against the reapers, he would still want to either use the research in some way or destroy it. He could try to use it to give the soldiers an edge to help in the fight or give it to medical specialist to develop new treatments. In the hands of an enemy, however, it could cause a potential distraction and divergence from the main fight by diverting resources to fighting an increasing threat caused by the research. For example, a terrorist group might use it to embolden their members and increase the frequency and scope of their attacks, as well as propaganda to increase their numbers.
This is just one starter concept used as an example. Bioware's writers could probably make the impact and significance even better than this. As said before the main importance of the mission would not be the hybrid child, but rather the military and medical applications. The option would appear when the right conditions are met, and even then the player still has the choice of what to do. So the people who want the hybrid child can have it, those that don't can play the mission and use one of the different options, without even seeing the option for the hybrid child. If you're that opinionated, this still works for you. Otherwise you are just arguing against other people having it, even if it doesn't affect your gameplay.
As for the genetic thing, I get why it is complicated. I've had a science person explain why it is difficult. Contrary to what people say, I don't view the issue as continually being pointed out. It is mentioned in the codex and in brief side conversations, but I definitely wouldn't say that it is pointed out more than anything else. This point seems exaggerated to an extent. It is used to suggest that this would be impossible, regardless of anything that could be done. The idea is that we don't know any way to make this work, so the science fiction couldn't either. Biotics, FTL travel, the asari, and overlord, to name a few, aren't possible by known means right now anyway. You can argue that they are possible in theory, but it is still the same concept. Just because something isn't possible now doesn't mean that it won't ever be, we are always finding that out. That way of thinking is far too short-sighted.