We have too many people on the planet. Carbon emissions is only one of the many huge environmental problems we have. Creating new consumers to the planet is clearly the most devastating act for the environment what individual people can do.
The number of people that can inhabit the planet without destroying the environment is highly dependent on individual choices and (maybe even more) government policy. Government policy is determined by voting power, which is in turn determined by the values of people weighted by population. So absolutely it matters if the pro-environment folk advocate removing (to some degree) their own voice from future populations of voters while anti-environment folk do not.
The most devastating act that individual people can do is to not pass on positive, lasting values to the next generation. Because you're not going to stop anti-environment people from passing down negative values by doing so; you'll in fact be amplifying their stance. We must heal the Earth.