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 …
I’ve tended to use “female” when talking about both girls (children) and women. Calling a grown woman a girl always felt a little infantalizing, especially when that isn’t done much to men. At the same time, girls are not women, and I try to be precise in my wording. Female (and male) just make the most sense.
Ive heard this is in relation to some guys preferring women as girls sexually but it being less common for people to prefer boys sexually. Also iirc that weeb moe movement focuses on girls not boys.