Environmentalism is often at odds with human interests:

The human population is beyond a point that could be sustainably supported, but the means by which the humans would do the least damage, or damage the planet less quickly, often involve diminished lifestyles: living in smaller houses, restricting movement within smaller communities, using less modern conveniences, owning less stuff, etc...

The case is even more stark when we view the 3rd world... If the 1.3 billion Chinese people and 1.1 billion Indian people try to live like the 300 Americans... the damage to the planet would be inconcievable...

The struggle the environmentalist movement always has and always will have is to balance what causes less damage and what causes less inconvenience, harm, suffering, etc... to humans - which doesn't matter in a purely environmental sense.

This is why most environmentalists support reducing fertility. It would be easier to help the planet without hurting humans if there were less damn humans.