There is an interesting copunter-arument when it comes to euthanasia, (which you did not mention but ties in nicely with the consenting adults bit)

The libertarian / liberal view is bodily autonomy / consenting adults we should allow it.

The counter-argument however, is that by even introducing it as a possibility, you change how people interact with other possibilities. E.g. in my country the state pays for healthcare, and the money for euthanasia will certainly come out of the healthcare budget. So every year, some accountant will have to decide how much money to earmark for euthanasia, and how much for treatment. Killing people is a lot cheaper than healing them, so you will inevitably end up creating a pressure on some people to take euthanasia when they otherwise wouldn’t have (Because the money which used to go to treatment, has now been redirected to killing them)

I suspect you can construct a similar argument about kidney selling. As long as you have an environment where it is legal, it disincentives power structures from exploring the options which would make it no longer necessary.

A friend from Spain studying law explained why not only is surrogacy illegal in Spain, but why it is an actual crime to even have a child from surrogacy — doesn’t matter in which country you had that surrogacy.

The point here is that the surrogate mother, as you have pointed out, develops an emotional bond to the unborn child, and then is contractually compelled to give away that child. If you allow surrogacy, you are basically putting contract law above intuitive, inborn feelings of family relations. It is cruel. Really cruel. Unbelievable cruel. Taking a child away from it’s mother cruel, literally. There’s a reason that’s often used as a baseline for measuring cruelty. According to that friend, the surrogate mothers usually are completely devastated and need extensive psychotherapy (which they usually don’t have access to).

Furthermore, you’re basically doing the most sexist possible thing: Reducing a whole woman to her reproductive capacity. Not sure how much that latter point matters to you, but it certainly matters to me.

Like, yeah, maybe you could make the argument that surrogacy could be possible for a mother who doesn’t develop that emotional connection, but that seems such an outlier case so unlikely that it’s more likely that a desperate woman is lying about being able to do that than her actually not making that emotional connection. Moving the point of when the decision is made which family the baby will belong to after birth runs into the same problem. There’s a reason that voluntary slavery is illegal: Desperate people would do it (and have historically done it), and that didn’t make it right.

I may have gotten some details wrong.

Lots of people want their kids to get better grades. Lots of people pay for tutors or extra after-school education. You could directly pay your kids to get good grades. This seems strange and possibly bad, though I’m not sure why.

1. I had friends in the early 90s who were paid $20 per A on their report card. This was very foreign to me; getting good grades was considered a baseline expectation in my house (I did not study, do my homework, nor get good grades. I’m about 99% certain that $20 per-class per-quarter would not have motivated me to do any of those things).
2. There is some evidence (but I’m never sure of the quality of evidence in behavioral sciences) that adding an extrinsic motivation to something decreases intrinsic motivation. Thus the fear that paying kids for good grades would decrease their desire to perform well out of some internal sense of pride.