Yes, green sex - for married people.  "Natural Family Planning" (NFP) is a method of responsible family planning that does not involve the use of hormone pills, inserts, implants, latex, foams, spermicides, or surgical snipping or tying.  It can be used to help a couple conceive a child or, for serious reasons, delay her conception.

NFP involves tracking the obvious signs of fertility (ovulation) by a husband and wife.  These signs include: 

• basal body temperature,
• quantity, and type of cervical mucus, and
• position, size, and texture of the woman's cervical os. 

It's "listening" to the woman's body and it is completely natural.  All with 3 minutes per day effort, max.