Charlie, If you find the Scriptural references and the statements from the Church Fathers unconvincing, does the fact that contraception - not only abortifacients, but barrier methods and withdrawal as well - was universally condemned by Christians for almost 2,000 years, does that not at least give you pause?
But according to Deut. 25:7-10, the penalty for refusing to keep up a family lineage is not death, like Onan received. So it seems to me that the traditional interpretation that Onan was killed for wasting seed is the correct one.