An area of science he poorly understood. I agree he probably never should have even had a view on that since that wasn't his focus. Then again, most experts of the era he lived in may very well have been pro eugenics. I doubt it was all that uncommon. in fact I'm pretty sure it only started being really looked down upon after World War 2.
It also doesn't seem like he really pushed this idea anyway, it almost just seemed like a random thing that he thought. Hell the dude was celibate his entire life. (mentioned in my linked article/comic thing) So it not like he thought his genes were inherently superior. Or if he did, it would seem odd that he put no effort into passing his genes on to the next generation.
Biology/genetics wasn't, in general, very well understood area of study then, and this is a pretty small blemish on his otherwise awesome nature.
Also, IDK for sure but I'd assume he probably started talking about that when he was starting to lose it later in life.