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 [These Exit Times] (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 also 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.