Women were basically indignant about African Americans being able to vote before they were.
The 15th amendment allowed African American men to vote by stating that the “right of citizens of the US to vote shall not be denied or abridged by the US or by any state on account of race, color, or previous condition of servitude”. This took nearly one hundred years to be fully followed due to the restrictions put on black voters, especially in the south.
Women began to use this to their advantage basically by saying, “You’ll let these slaves vote before your own wives?” Remember, in 1870 pretty much everyone thought black people were inferior to white people and the idea that a former slave, an African American, could vote before women was outrageous to them (women). This also set off the women’s suffrage movement because women saw that racism was stepping forward so they thought maybe sexism might be resolved as well. Basically, women felt as though they should also be included in the 15th Amendment if they were already resolving one issue.