There are several USA-based organizations promoting reduction of population by voluntary abstention from parenting, particularly with a view to redistributing to people living now instead of hoarding material goods for unknown and uncaring future generations. Examples (1994) are The Voluntary Human Extinction Movement, Portland, Oregon, Childless by Choice, Leavenworth, Washington and Childfree Network, Citrus Heights, California.
The Gaia Liberation Front (also USA-based) supports all voluntary efforts to make humans extinct, but does not exclude the involuntary routes, such as involuntary sterilization, new anti-human viruses and genocide. As a rule, however, it regards war as an inefficient way of making humans extinct -- every quarter of a million humans killed represents only one day's growth of the human population, but more importantly because conventional warfare does a lot of collateral damage to non-human life.
VHEMT promotes voluntary cessation of breeding in order to allow the earth's biosphere to regain its good health. Failure to breed is presented as a caring alternative to involuntary violent extinction.