Groups of idiotic angry people ruins words all the time. It's not that the word female is offensive it's that it's used by a hateful group and hear it reminds us of that hate and some of us would prefer not to use it when referring to people. Language evolves and changes and culture dictates what they mean. The word unfriend is in the dictionary now. You are free to use whichever word you'd like. That's what freedom of speech is all about. However we are free to associate the use of the word female with incels and if you use it knowing that you'll get associated with them thru it's usage. You're free to do so of course but words are important and will dictate how people feel about you. Sorry I guess?