Other peoples babies make me suffer

I love kids. I love my kid, I love my friends kids. I think kids are great. I'd never be anti-family and I give all the props in the world to women who are on a 12 month + beatdown to carry then nurture that baby. I'm big into large families and having both mom and dad be around as much as they can. I'd rather mom be home all day, but sometimes they can't. I wish maternity leave was longer because it's crucial for mom and baby to have that bond. However:

Why do I get penalized for another woman's baby? A co-workers maternity leave can put my work/life balance completely off-balance and it'll also have an effect on my income. I hate to be honest, but if I was an employer I wouldn't hire a female of child bearing years no matter what. Why would I want to hire, train, rely and depend on this person for them to "go get pregnant" and leave me high and dry trying to run my business while they're out on leave? Then they have a free pass to come back.

I have an assistant who I depend upon heavily because I'm depended on, by my company, to produce an above average sales volume. Now that she's gone I'm left with a temp or myself to get this job done. It's ineffective, inefficient and my production suffers. If my production suffers, my paycheck suffers. Why should I get penalized for that? She gets paid while on maternity leave, but my paycheck suffers for trying to do the same job?

I can see why women weren't in the workplace decades ago. Of course they can do the jobs that men can do, but you can't rely on them if they are in-and-out of work trying to have kids. That's not fair to companies or co-workers.

I don't have a solution nor would I make a stink to change the rules. It is what it is and I'll just deal with it, but if you think about it it isn't very fair.