Murderbot blends as a human. Can you remember a specific passage where it's a dude? It could be your brain playing tricks on you.
I'm a man and I'm guilty of assuming a male gender for characters where the text says "a guard", "a doctor", "a soldier", etc. When I was told some readers thought Murderbot was female, I re-read several passages and nowhere does Martha Wells state its gender either way (even by stating Murderbot passes for a man or woman).
Mainly I think we came up with that mindset immediately upon reading the first novella and never updated it.
I'm pretty sure it's never stated it looks like either a man or a woman. I mean, if it can pass for human it must like either or at least androgynous, but Martha Wells never tells us which.
What I do find interesting though is how many people chimed in here with a very strong sense of this detail but it was all a mix of responses! :)