What does it mean if someone won't make kids with a mentally ill person?

That they're sensible.

It means that person may not be fit to be a parent. It also means there's a good chance the children will inherit that mental illness.

Because if they won't be able to take care of the baby. Because they won't be able to have sex at first place.

Their choice

Eugenics.

The only way they can control that is by refusing to have sex with them. If they consider the person’s mental illness a problem, why wouldn’t they ?