Habitat destruction and subsequent loss of ecological diversity (based on a long-term view of several thousand years).

Note: I'm ok with abortion, and also with having kids - I endorse the idea of keeping the global population relatively stable until we can figure out how not to screw over the environment while we're busy making more stuff for ourselves. I don't endorse any drastic enforcements to that, because we seem to be doing ok on that front (peak child et al)