Clarifying the Sin of Onan

When talking about the reproductive health (RH) bill, barrier contraception (e.g. condoms, cervical caps), masturbation, and the withdrawal method, the sin of Onan is sometimes linked to the issue by anti-RH people, especially by anti-RH Catholics. They say that barrier contraception, masturbation, and the withdrawal method are connected to the sin Onan committed. 

In this post, we are going to clarify what really is the sin that Onan committed.