I think you are framing it incorrectly as discrimination.

If one stretches such an analogy, one could reason that paid sick leave is discriminating against healthy people. Not many people would agree with that one.

As you've stated, a sensible medical policy for everyone would be good. I would imagine that Google has a one.

Absolutely I agree that paid sick leave is discriminating against healthy people. If you're going to pay people to not work, everyone should be equally eligible.

Adoption isn't a debilitating medical condition so why would you get medical leave for it in addition to parental leave?

I think OP means that birth mothers get a little more time than fathers and adoptive parents. Having a baby is rough on the body, and there's a lot more to the physical recovery afterwards than most people realize. For example, if you have a c-section, it takes approximately 6 weeks to recover from that, and there are certain physical side effects that make it more difficult to work from an office until recovery is complete.

So that part is more like medical leave than parental leave (even though you're also parenting at the time).

Adopted parents aren't excluded. You get the leave whether you are becoming a parent through adoption, surrogacy, child birth or fostering.