It seems obvious to me... but I will explain it.
160 years ago, almost everyone who was against slavery in the US was a Republican. The people who call themselves Republicans today are something different. They talk a lot about freedom, but there are lots of people who are against slavery today who are not republicans.
100 years ago the suffragettes were working to get the right to vote and help women get equal rights in society and in the workplace. The people who call themselves feminists today are something different. Today's feminists talk a lot about equality, but there are lots of people who believe in equality today who are saying "I am not a feminist".
I am not a Republican. I am not a Feminist. Interestingly, the Republicans feel the need to make vulgar personal attacks for me daring to disagree with them.
I can't think of many groups.. other than feminists and fundamentalist religions, who use such ugly insults against people for simply choosing not to join them.