Couple of things, @missshortie89:
First, why is 'cis' offensive? --One reason is that in order for 'cis' to make any sense in locutions such as 'cis man' or 'cis woman', it would have to be the case that it's possible to change sex. But it isn't. So it doesn't. And blithely assuming the person you're speaking to is the sort of person who talks, or accepts, nonsense is offensive. Do you see now?
Secondly, yes, it's nice when things like this can get discussed, as you say. One important aspect of discussion involves understanding what one another is saying. I confess I don't understand what on earth you mean by 'gender'.
Sometimes 'gender' just means 'sex'. OK. Other times 'gender' means something like 'the collective attributes or traits [culturally] associated with a particular sex'. Again, OK. But you seem to use it to mean something else. What? (Your Yale thing didn't help - you needed to know what 'gender' meant to be able to understand its explanation; the word 'gender' sneaks into the explanation. Hmm.)
Please, @missshortie89, can you try to explain what you mean by, for instance, being 'born in' a gender. I just don't understand: of course it would make sense if you just meant 'born a certain sex' ... but if so, surely you would have said that? And the other sense I mention above just doesn't fit at all. So you can't mean that. What do you mean, then? What sense of 'gender', non conterminous with 'sex', makes sense of being born in a particular gender?
To be clear, @missshortie89, I think you are talking nonsense, even though you don't realise it. Discussion might help you see more clearly. (And, likewise, I realise I may be mistaken.; if so, discussion might help me see my mistake(s).)
[Of course - this is the internet, after all - you may just be playing the faux naif, as I think someone else mentioned. Your tone suggests that, for sure; rarely is such disingenuousness real. If so, it's unlikely you'll wish to discuss matters in any way openly. But, well, take the benefit of the doubt. (And if you don't understand this parenthetical commentary, just ignore it and accept my apologies for being an old cynic).]