Do Jehovah's Witnesses allow contraception, what forms do they allow, and what are their arguments for it?

Questions specifically on the Jehovah's Witnesses faith on contraception: 1. Do Jehovah's Witnesses allow contraception? 2. What forms do they allow? 3. What are their arguments for allowing (or not allowing) contraception? On question #2 I am mostly curious on what the Bible describes Onan as doing in Genesis 38:8,9 . Is that allowed or not and why? Genesis 38:8,9: > Then Judah said to Onan, “Sleep with your brother’s wife and fulfill > your duty to her as a brother-in-law to raise up offspring for your > brother.” 9 But Onan knew that the child would not be his; so whenever > he slept with his brother’s wife, **he spilled his semen on the > ground** to keep from providing offspring for his brother.