I've worked in IT for 25 years; I've been a sysadmin and all other sorts of things during that time (I now run security for a company). I've worked in some really conservative places, and I've seen sexism in action but never let it affect my choice of work. I would absolutely encourage women to enter the field. If you ignore any sign of sexism and bulldoze your way through it, it tends to go away (or the sexists nurse their quiet grudges, which is just fine with me). Don't go looking for it, though, and don't spend every day thinking "OMG, I'm a GIRL!!" Just do your job as best you can and leave your gender at home.