Eugenics - it makes sense on the surface, right? You breed the race for good traits. If someone has 'defects' they don't breed and we become stronger. Its like evolution, or breeding livestock. It all sounds like a great idea...until you think about it. Because then you either have to have a population that is totally onboard with asking permission to breed or you have to enforce it. Neither solution sounds like a fun society to live in.
Now the Sanger quote, I don't think it was racist. I think racial elements means the elements of each race. Like breeding dogs. "On the contrary, the most urgent problem today is how to limit and discourage the over-fertility of the mentally and physically defective."
"keep the doors of immigration closed to the entrance of certain aliens whose condition is known to be detrimental to the stamina of the race, such as feebleminded, idiots, morons, insane, syphilitic, epileptic, criminal, professional prostitutes, and others in this class."
You also need to remember what it was like before birth control. Its easy for us to forget. Some families had more children than they could take care of. If you literally cannot feed your family you might make some very poor decisions. Accidental pregnancies were pretty much only avoidable by abstinence, and that ain't happening. So, while her belief in birth control was based on Eugenics, people are generally more complex than one idea.
"Usually this desire [for family limitation] has been laid to economic pressure... It has asserted itself among the rich and among the poor, among the intelligent and the unintelligent. It has been manifested in such horrors as infanticide, child abandonment and abortion."
"It is apparent that nothing short of contraceptives can put an end to the horrors of abortion and infanticide."
Even non-race based eugenics is scary though. Who decides what is or isn't merit? I'm actually reminded of a Star Trek: TNG episode where Geordi is talking to a Romulan and tells him he was born blind. His response was to say that this was why Humans were weak. "And your parents let you live? You waste time and resources on defective children."