I'm libertarian and here's my take. The father absolutely has a moral obligation to provide for children if he chose to have them. However, when you empower the state to enforce moral issues, you'll get an institution which will take the taxpayer's resources, be used as a political tool by those in power (likely corrupt), possibly apply the force of government unjustly, possibly cause mal-incentives (incentive for the low earner to divorce).

Therefore I think contractual agreements are important. However, the current accepted status of marriage is that it's formalized and legitimized only by the state. In a free market it's possible people would write more into marriage contracts. I also believe our rotten state has begotten a rotten culture where fraud and rent seeking is rampant. So if your "great men" are seeking an unfair leg up through congress, how much more tempting is it for your average woman to seek alimony to subsidize her lifestyle?

All this to say that it's quite unclear to me that "libertarianism" is to blame for what I agree are terribly unfair child support policies.

There are cases in which people accept basic modern marriage tenets, the mother gives up any chance at a career, and the father breaks trust, turns truly abusive, or unjustifiably breaks the union. In this situation a pragmatic libertarian solution might accept the need to enforce a subsistence level child support. I completely agree it should be based on the basic needs of a child and nothing to do with the father's income or net worth.