In the past, most states provided that custody of children of "tender years" (about five and under) had to be awarded to the mother when parents divorced. In most states, this rule has either been rejected entirely or relegated to the role of tie-breaker if two otherwise fit parents request custody of their preschool children. No state now requires that a child be awarded to the mother without regard to the fitness of both parents. Most states require their courts to determine custody on the basis of what's in the children's best interests, without regard to the parent's gender.
As it turns out, many divorcing parents agree that the mother will have custody after a separation or divorce and that the father will exercise reasonable visitation. This sometimes happens because the parents agree that the mother has more time, a greater inclination, or a better understanding of the children's daily needs. But it can also be because fathers presume that mothers will be awarded custody or because the mother is more tenacious in seeking custody.
If you are a father and want to ask the court for physical custody, do not let gender stereotypes stop you. If both you and the mother work full-time, and the kids have after-school care, you may be on equal footing. In fact, if you have more flexible hours than the mother, you could have a leg up. In any event, the judge will look at what's best for the children. So if you think that you should have primary custody and that you can persuade the judge that it's in the kids' best interests, you should go ahead and ask for custody. If you present yourself as willing and able to parent, it will go a long way towards challenging any lingering prejudice against you as a father.
If you are a divorced dad looking for practical strategies to maintain good relationships with your children, check out Being a Great Divorced Father, by Paul Mandelstein (Nolo).