Much of the problem people have with Sanger has to do with her embrace of one cause. These next two paragraphs are quoted from the article in question:

Sanger’s eagerness to mainstream her movement explains her engagement with eugenics, a then widely popular intellectual movement that addressed the manner in which human intelligence and opportunity is determined by biological as well as environmental factors. Hard as it is to believe, eugenics was considered far more respectable than birth control. Like many well-intentioned reformers of this era, Sanger took away from Charles Darwin the essentially optimistic lesson that humanity’s evolution within the animal kingdom makes us all capable of improvement if only we apply the right tools. University presidents, physicians, scientists and public officials all embraced eugenics, in part because it held the promise that merit would replace fate — or birthright and social status — as the standard for mobility in a democratic society.

But eugenics also has some damning and today unfathomable legacies, such as a series of state laws upheld in 1927 by an eight-to-one progressive majority of the U.S. Supreme Court, including Justices Oliver Wendell Holmes and Louis Brandeis. Their landmark decision in Buck v. Bell authorized the compulsory sterilization of a poor young white woman with an illegitimate child on grounds of feeble mindedness that were never clearly established. This decision, incidentally, was endorsed by civil libertarians such as Roger Baldwin of the ACLU and W.E.B. Dubois of the NAACP, both of whom Sanger counted among her supporters and friends.

(bolding mine)

To be fair to Margaret Sanger, she had no idea of what horrors others would commit in the name of eugenics, and she never proposed anything like the mass murders committed under the Nazis infamous T4 Program. But she had let the eugenics movement her voice in its early years (to the extent of writing a pro-eugenics book titled The Pivot of History in 1922) and thus must bear some blame for the excesses that followed.

Sanger wasn’t motivated by racism, not at all. But the use of some the laws she championed against blacks at the very least doesn’t look good, and it was made worse by the fact that she never really gave an accounting for her early support of eugenics. To be fair, by the time she wrote her autobiography in 1938, the embrace of the rhetoric and some of the goals of the worst of those who had promoted eugenics by the Nazis meant she could hardly give an explanation without providing her foes with something to discredit her with.

So while Sanger wasn’t really racist, her association with a movement that did some truly horrible things left her sometimes looking like one from the vantage point of our own time.