It might be rare but it exists. I don’t think she said it was the “norm,” whatever that means. A small minority is still a part of the whole. In fact, if only one person in the entire world existed with XX chromosomes and a penis, it would invalidate what he was saying, that chromosomes = sex (or at lease primary sex characteristics). The only effective rebuttal to what Sara said would be to show that *never* happens, and even you admitted that it does happen. That’s what “rarely” means: it happens, but not very often.