I think, knowing Aella, that she is endorsing the statement in her tweet. But yes technically she should have worded it differently considering what she was responding to.
idk i view having kids as like 'starting a company' - you agree with someone to commit to working on an external project.
And this just requires both people give enough attention to prioritizing kids, it doesn't mean you have to forbid extrarelationship interactions.
If your partner impregnates another person, your children will likely be financially impacted for the rest of their lives, and that child will be denied the attention of two parents. This isn't an edge case, it happens all the time
I guess women could try to require that their parents only sleep with other people who are demonstrably sterile, but nobody actually does this and everybody just assumes the risk of a BC failrure/manipulative partner