I get the likely reason why you don't find it offensive, but I also get why plenty people do.

Note how the complains are usually towards the usage of "female" as a noun, not as an adjective. That's because of a small quirk of English, that marks adjective nominalisation rather heavily. To show it with a non-offensive example:

• I got two cats. *The young is a tabby, and *the old is *a bicolour.

That likely sounds fine in the other language[s] that you speak (as it would do in my L1 and L2), but it sounds weird for English speakers - they'd expect "young", "old" and "bicolour" to be followed by nouns, not to be treated as nouns.

As a result, when you "promote" an adjective to a noun, people usually take it as creating a category aside from whatever category the relevant entities were formerly assigned to. And if the former category was "human beings", the nominalisation becomes dehumanising.

Another example [now offensive] to highlight this would be:

• "Alice is gay" - most people wouldn't raise an eyebrow to that
• "Alice is a gay" - since the usage of article forces reading "gay" as a noun, it suddenly sounds dehumanising.

The same process actually does apply to "male"; the main difference is that men aren't seen as a disfavoured group by society, and people often take that into account when judging the offensiveness of an utterance.