Jo: Your chauvinist crap. You think women can't do the job.
Dean: Sweetheart, this ain't gender studies. Women can do the job fine. Amateurs can't. You have no experience. What you do have is a bunch of half-baked romantic notions that some barflies put in your head.
Amateurs can't do the job? Given that there is no school for hunters that we are aware of, every single hunter on the show started off as an amateur. Dean wasn't just an amateur when he started - he was a kid. Jo is new to hunting, but she's an adult who has done her research. Given that Dean calls her "sweetheart", I think her gender is a bigger part of why he's treating her the way he is than he wants to acknowledge.