This is a question and a statement I hear from female clients quite frequently. To date none of my female clients have gotten "muscly" or man looking from lifting weights. However it is quite a common concern for women.
Here is why they do not have to worry about getting too much muscle. They do not possess the hormonal profile to grow muscles like men do. Muscle growth has many components to it, with arguably the most important one being testosterone production. While women do produce testosterone, its at roughly the same level of prepubescent boys (which is roughly a tenth of what grown men produce). In the absence of testosterone one does not have to worry about becoming "muscly".
Those women who compete, and do in fact build muscle are not doing it by accident. They are following a program designed to build muscle that focuses on overloading the muscle with heavy weight, taking supplements for building muscle, and some take either testosterone or they take a hormone like supplement to assist in eliciting this muscle growth. These programs and workouts are far different than one's used for overall fitness.