I don’t think “female” is offensive.
I thought it was offensive because some (supposedly female) users said it was offensive; but I didn’t understand why my female friends used the term (female).
I found another saying that it wasn’t offensive, and I immediate copied that belief.
LGBTQ+ politics is confusing …
No, it’s just an adjective for gender. But if instead of saying something like “my best friend is a woman” you said “my best friend is a female” it sounds like you are an alien talking about some kind of specimen.