DLion, your post only makes sense if you assume the parents have no inherent obligation to their children. On what basis could you possibly make such a claim? Bringing a person into the world is a very serious matter, and the idea that you shouldn't be responsible for your decisions is quite frankly bizarre. Raising a child costs money. In deciding to have a child, you are agreeing to provide for that child, financially so is just a part of that. There's nothing unjust about that, just as there's nothing unjust about having to make payments on a car you signed a contract for even if you decide later that you don't like the car (or the contract). Essentially your argument comes down to "it's not fair that I have financial obligations that I can't walk away from at any time".

Even spousal payments can make a lot of sense in your strict contract-based world. A marriage is intended to be a for-life arrangement. One of the benefits of such an arrangement is that it allows for both partners to coordinate their actions in a way that wouldn't be possible or desirable if each was acting individually. One common example of this is that one spouse gives up some or all of their current and/or future earning potential in or to provide non-monetary benefits to the household. One partner is willing to bring these non-financial assets to the table with the understanding that the other partner will instead contribute more on the financial side of things. Thus, the situation of the couple is optimized locally, not globally.

If that arrangement is violated, then one partner is harmed financially, as they were willing to take reduction of personal financial status in order to achieve a more desirable overall situation for the couple. Since this "contract" has been violated, compensation is made to offset the personal financial loss incurred by one partner by agreeing to the arrangement.

In summary, I would suggest that if you cannot think of an arrangement between two people in terms of pooling resources in pursuit of a common goal, knowing that neither partner will contribute exactly 50% on all dimensions, then marriage probably isn't a desirable arrangement for you anyway. "Keeping score" on every detail is a recipe for failure in a relationship imo.

Yes, and? The fact that it's more expensive for you and your partner to live apart doesn't free you from financial obligation to your children. Yes, both partners will have a lower standard of living apart than together....so? That's not unjust, just cause and effect.