Not an economically viable strategy for any company looking to stay in business more than a year or two.

Not for a good company. I've had several employees go on maternity and paternity leave. I've given them respect, and time, and money, in order to make this time of their lives as unstressful as possible.

I've done it because it's the right thing, but these people remember being treated well. They've come back to work and done very well. They feel safer knowing that their day job explicitly supports their families.

The time someone needs off work to care for a new family is a drop in the bucket compared with a whole career, or even a few good years spent at one company. Optimize for the humans, and for the long term.

No. The only real way to avoid this is to just not hire folks who can birth or father children. You'd be mostly safe hiring folks over 45 (Though men over 45 would be more risky than women). This probably isn't a good strategy for an employer.

Marriage doesn't lend itself to children: Having sex does. Having a stable relationship does. Adoption does. On the other hand, lots of married folks don't have children, on purpose or by circumstance. There is no real way to sort folks out.