Yeah, to clarify, it's not unethical to
want a lot of kids, or to have them when it's socially beneficial. But if you're living in a post-industrial society with decent medical care, it's unethical to actually have more than 2-3, both because of the unnecessary weight on common goods and limited resources (both personally and societally). They're probably not going to die young and some or all of them will have children of their own.
Look at the costs. It's fucking expensive to raise children, especially in the first world. We're talking six figures from birth to college graduation. The average middle class family can bear that cost for maybe two or three kids if they save hard. A lower-class family with steady income can manage maybe one or two. Any more than that (or multiple children for people at welfare levels of poverty) and you're going to have to play triage with family finances, either giving all of your children suboptimal care and resources or denying it to some while giving it to others.
That's just the personal/familial impact, of course.
If you're in a post-industrial society the shotgun approach to reproduction might still feel natural because it's instinctual, but it's actively detrimental to everyone involved-you, your children, and the society in which you live.
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In a less technologically & medically advanced society where a lot of kids are going to die young and large amounts of manual labor are necessary to run a family, it makes sense. In a post-scarcity (or even just multi-planetary) society, who cares-you can support as many as you want in the former, and there are plenty of places that will need booming populations in the latter.
But in our current situation on Earth right now, we're already well past the point of being able to support the global population at the contemporary standard for comfortable middle-class living. And that with the vast majority of Earth's population living
below that standard. Even if you adjust for inefficiency, wastage, and killing off all the super-rich for good ol' resource redistribution you can't make the two ends meet. Our ideal right now is actually a
decreasing population in order to decrease pressure on limited resources and reduce environmental damage, while also raising average quality of life. But people don't like that shit, so we talk about station-keeping at a slight net increase with two as a semi-tolerable compromise.
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The tl;dr is: fukkin adopt ya shitlords. If you have old-fashioned attachment to blood continuity, have one and adopt one or two, as long as you're not going to be one of those jackasses who treats blood-kin different from adopted kids.