When I wrote my post on the locked thread created by Hollywood, I went to bed, and woke up to find that it was locked. There was an active discussion about it so why not discuss it further.
Eugenics shocks a lot of people but I think it is inevitable, now that nanomachines offer promises like getting rid of cancer. My reasons for why it will happen:
1) Dwindling natural resources - Things are taken out of the Earth far faster than they are being replaced. Something nobody thinks about is water. Confined aquifers (underground water sources used for irrigation) are being drained quickly. For some, for every 1 cubic meter of water recharged into the aquifer, 12 cubic meters are getting taken out. This means that underground water will soon run out, creating mass starvation, unless someone were to find out a way to grow edible plants for mass consumption in salt water or no water at all.
Eugenics would play a role here because you could have more scientists and engineers to fix the problem and less people using the remaining precious natural resources. In other words, the worthy people will be the last ones to use it.
2) Controlling epidemics - AIDS isn't going away and in fact, it is spreading to other nations. A forced sterilization and quarantine program can significantly cut down the AIDS transmission and this includes rape. AIDS would not go away but turn into something like smallpox where there are a few cases here and there but nothing significant. Note this doesn't mean targeting specific ethnicities or orientations. Just those that are infected. Besides, I doubt AIDS is the last epidemic to happen to humanity nor is it probably the most severe. Why should more virgins get raped or people get the infection because they were never informed of their partners' condition?
3) Reducing violent crimes and sex crimes - If a pedophile rapes a child, forced sterilization. While it is unfortunate they have the urges to diddle kiddies, that doesn't mean they should breed either. Chemical castration is a fence sitting answer.
The ethics issue here is that in the past, a lot of times race was the determining factor. I don't think it should be now but as we continue to advance technologically in society, we can do better than simply live and let live. That plan isn't working.
Note that my reasons for eugenics may differ from others.