It’s sexist if you don’t look further into the claims, instead just relying on your immediate assumptions about them being false.

If you immediately assume women are lying about experiencing sexism, and you don’t look into it further at all, and your reasoning is based solely on them being women as opposed to men, then yeah I’d say that’s pretty sexist. I’m not sure how someone could think otherwise.

Other types of sexism include disbelief when a woman explains their experiences and baselessly denying evidence they present to support their claims.