I agree that these "musts" centered around marriage are really unhelpful and coercive. Some parents and cultures have promoted that. Though it is good to have children, due to a lot of factors many marriages don't result in offspring. And back to the original topic, Onan was "guilty" of coitus interruptus. The big bad judgment was centered around being given a fairly easy task of consummation of the natural sex act. What was his problem to not do that? A lot of people "fool around" with sex for years before being willing to have a baby. Personally I think that abuse in marriage is the big problem, not giving each partner a voice in decisions making. Unnatural and disrespectful actions in marriage undermines the relationship.