That's just the problem... the creation of a child is a one-off, chance event. Even giving birth to the child is a singular event. Someone can decide they want to "have a child" or "give birth to a child," and once that event is complete, they're satisfied... but then they're stuck with, well, a CHILD for the next eighteen years. And in far too many cases, they decide that, since they've gotten what they want, someone else can handle the child.
Maybe they selfishly decided to fulfill that biological drive when they had no job, no education, and absolutely no means to support that child. Does that mean it is the PUBLIC'S obligation to get them a child? That's the problem--we're putting the burden of proof on the wrong side. Instead of "Why shouldn't this person have a child," we should be asking, "Why should I have to pay so they can have a child (which I will then continue paying for)?"
If you can't support the child (financially, materially, emotionally, etc.), it's YOU that needs the pretty compelling reason why everyone else should fork over the cash to support your habit.
A mentally-handicapped person could decide, "I like babies, I want one," or hell, even just "Sex seems fun, I want some." Ta-da, they get someone in the same group home pregnant, or get pregnant themselves. Now what? That wasn't an informed decision, but they made it, it's made, and now what? All they had to decide to do was have sex, and everyone else who has to deal with all of the consequences gets no vote?
It's not about freedom, it's about the impact of that freedom. If your exercise of freedom can be, for all practical purposes, self-contained (that is to say, you can pay for any expenses of time or money incurred by that exercise of freedom), then yes, it's YOUR freedom. If not, is it really YOUR call? Am I allowed to spend your money simply because I want to? Hell, at least with the government, I can try to vote them out of office if they spend my tax dollars in a way that I don't like... but with these folks, we have NO recourse.
I've seen women who've already had THREE kids taken away by DSS, for neglect and just plain inability to support the child. And this is AFTER a few years of the government forking over welfare dollars, WIC, and every other entitlement program. All of that money thrown at her, they STILL take the kids away, now they're being raised by the state (us), and SHE'S PREGNANT AGAIN. And she can keep doing it until what? Until she's put in jail for breeding? (Can't) Until she's told to stop it? (Can't) Until she's obligated to take birth control? (Can't)
The economic reasons behind all of this are valid. It's not about some indirect cost-benefit analysis about the "greater good" as determined by suits in an office somewhere far away. It's directly costing us money and resources today, here, and now, raising the kids of people who were unfit to be parents to begin with. It at least bears some consideration, without being dismissed as "Nazi."