It's equivalent to pronatalists banning suicide, in my view.
Suicide only involves you as an individual, no one else his hurt by it, at least not directly. The same cannot be said for procreation. The parents are the ones who make the decision but in the end it is the child who has to live with the consequences of being brought into existence.
Although I feel very strongly that I want to die and that death should be easy for me, I have to recognise that there are other people who don't feel that way at all about their own lives.
Maybe you are mistaking antinatalism with promortalism? Antinatalism merely assigns a negative value to birth. The question it answers is whether or not a life should be started, not if it should be taken. So antinatalism has no problems with happy people.
Regardless, even if people are happy with their lives that doesn't give them the right to impose life and create a person who has no way to give their consent. Saying "but it's the decision of the parents so it's okay" is like saying "it was the decision of the rapist to violate someone, so it's okay".
No-one was ever hurt by nonexistence, the same can't be said for existence. The potential misery that could befall anyone on earth is enough to call the creation of another person into question.
I feel that supporting antinatalism is like laying claim over everyone's lives, including those who would live despite all the suffering.
The ones who would've wanted to live despite all the suffering won't be there to grieve their nonexistence since they were never conceived to begin with.
EDIT: it's also worth pointing out that Antinatalism doesn't necessarily entail things like forced sterilization or any of that sort. There are many antinatalists who don't agree with such practices, most just want to make the philosophy public in order to convince as many people as possible to voluntarily refrain from giving birth.
Antinatalism as a philosophy only has one single statement: that procreation is unethical. That's it.