Or to women who seek "an alternative arrangement" to carrying every pregnancy to term?
Really? You really went there? Why don't you explain to our viewers why you think that's congruent, and why you're not just being an asshole. We're talking explicitly about contracts for the care of children and disposition of property, as illustrated by this example. Try to focus.
Your explanation for why the sperm donor should be on the hook for support was that it was "the default, traditional, de facto state of affairs". There is nothing in there to suggest the argument can't -- or shouldn't -- apply to pregnancy vs abortion. Maybe you should be more careful before you throw out arguments based on unthinking, blind conservatism. That is what conservatism really is: A belief that we should leave things as they are because that is how they have been. When conservatism is the only rationale you offer, don't be surprised when someone asks if you apply the same logic to other conservative bugaboos.
But there's no justification to argue that only the people who support a certain thing should be taxed for it.
Sure there is. They're the people who want the thing (government support for the children of the poor), so they should pay for the thing. That's not a new idea. Maybe you've heard of this thing called charity, where people who believe in a cause that doesn't benefit them give money to support that cause. As an added bonus, they can give as much money to the cause as they choose.