Just being a majority on a certain ethical issue doesn't make it wrong.
What they mean is, historically, the majority has not always been correct when it comes to ethical issues. That's different than arguing that majorities have always been wrong.
Going to look back on modern factory farming (the source of the VAST majority of meat eaten in America) as something so abhorrent (Both for the process of it and the external costs) we’ll be embarrassed to tell our grandkids about it. Although as I’ve been downvoted for saying here before, if you’re having kids and grandkids, you’ve already done uncorrectable damage to the environment so go ahead and eat meat or whatever. Doesn’t matter at that point.
I hope (but doubt) that killing animals for food (whether by hunting or smaller-scale farming) will also become completely unacceptable outside of very specific situations.
There’s absolutely no shortage of people who a) will think this is bad advice that doesn’t apply to them because they are very smart, b) just don’t care, c) don’t have access to birth control, d) just really want a kid, etc.
Evolutionarily speaking, not having kids just leaves the planet to those who does.