Might help also to describe what you think feminism is, since it’s one of those terms that is overloaded.
I once had a physical therapist tell me she wasn’t a feminist because she thought women couldn’t be as physically capable as men when serving as soldiers, and seemed to believe feminism requires treating women exactly like men.
I told her I was a feminist because I believe in equal rights for men and women, an idea she did not seem so opposed to.
I think the reflex naturally makes sense, but from the people I personally know to be outspoken and definitionally feminist, it’s more like calling yourself a feminist says you explicitly side with the feminist cause. Sort of like saying you’re anti-racist rather than identifying as someone lacking racism, which is actually a farce when we’re all biased.