Don't worry, it's not 80%! My sister did so much research on this after her second boy, and although you start out slightly more likely to have a boy (not even as much as 51% statistically) and your chances of another boy go up slightly with each boy you have, it's nowhere near enough to be at 80% after two boys!!! It's something like 1% with each additional boy. I distinctly remember reading that the difference is minimal and your chances are always pretty much 50-50 no matter what you've had before! People do tend to give the impression that once you've had a boy or two, you're unlikely to get a girl, but that's not true - you're just a BIT less likely to get a girl, that's all. My aunty had three boys, then a girl. Then another girl! Defo possible.
