It doesn't really matter what the reasons are (and, frankly, having worked in the past for a couple of <10 employee businesses - you're doing something pretty wrong if one person being on the piddling amount of SMP you pay, and absent for perhaps a year, is causing the business to go to the wall), the fact is that what you describe is exactly the gender inequality that feminism is fighting to remove.
You're ruling out hiring a woman on the basis that she's a woman. Because some women have children.
Lots of people in this thread seem to think there's no such thing as gender inequality, or that it's just men who are suffering it. You take a different view - that gender inequality is perfectly excusable. Truthfully, you're far from alone - which is why such inequality exists.