I keep reading threads like this and have this argument at the back of my mind that I am struggling to articulate.
Lets assue every pregnancy involves a man and a woman as a couple (I know lots don't but enough do to make this a valid argument I think). If you employ a woman and she gets pregnant you may end up paying more than her salary to hire a replacement, there may be issues around handover of tasks and clients etc. So it can cost.
However, as each baby belongs to a couple, if you have a man, you are effectively getting the same outcome (a member of your staff having a child) 'free'.
So on average (unless you hire proportionately more women than men) and unless you consider the human race should stop reproducing, it evens out.
I know the word 'naive' will be used in at least one of the first 3 replies, but can you at least see my logic? If the couple were employed in the same small company it becomes a bit clearer.