Place yourself in their shoes.
How would you like to be deprived of what everyone else routinely enjoys and brags about, not because of something you do to deserve it but because of something you are and cannot change?
Humans are not soulless, mindless bacteria. Humans are sentient. Life is life; life is serious; life is not a game. Very few are genuinely stupid and fewer still are genetically so.
Besides, evolution doesn't work like that—humans must be fit for survival just as any other species. Evolution does not make exceptions. Evolution does not play favorites. Idiocracy, amusing as it is, is fiction, and if you still believe in degeneration or other such pseudoscientific hogwash, you don't deserve to reproduce any more than the people you judge.
I should also point out that books such as this aren't there to teach people how to reproduce. There are instincts for that. Even a pair of complete mouth-breathing morons will eventually figure it out. What's not so simple is how to make it fun. There are no instincts for that. Evolution does not select for that, either. Thus we must use the intelligence nature gave us to study the topic and write books to aid our fellow man.