He did not do the ritual of not taking his brother’s wife, and bear public shame; instead took his brother’s wife and then did not perform the act of giving her a child. Therefore, he broke the vow.
This example is why doctrine should never hang on a single historical narrative. It is akin to bad case law.
Let’s look at this another way: what if Onan never had a relationship with his brother’s wife? Would he not have violated the duty to bear his brother a child? And is it a sin to intentionally have a childless marriage, regardless of the means?
And as for the other items: fornication and sodomy are outside of marriage, despite what some of your clergy claim.
Keep in mind that NO Christian leader or denomination approved of contraception before 1930, when the Anglicans broke ranks with all of Christendom at their Lambeth conference. According to John Kippley (whose research you really should look up), not a single Christian writing between the 1st and 19th centuries saw Onan's sin as other than perverting sexual intercourse, wasting his seed upon the ground. This is a Christian consensus.
Was that consensus founded ONLY on the fact that contraception is mentioned only once in the Bible, when it is condemned? No, it's also founded on much broader and deeper principles of the Christian understanding of God's holy purpose for marriage.
Fertility is always described Biblically as a blessing and a gift of God. Never once in over 23,000 verses of Scripture is marital fertility described negatively; not once is the impairing of fertility described as a tolerable thing.
Anyone who reads this and does not consider it self-evident, is encouraged to go ahead and use their Biblical concordance and see that it's true.
This lines up perfectly with Natural Law, which sees fertility as a natural sign of health, and therefore the sabotage of fertility as a harm. Thus Natural Law teaches that intentionally impairing fertility via any deviate intercourse included contracepted intercourse, sodomitical intercourse, intentional maiming of sexual competency via sterilization, castration or sex-reassignment surgery, is a wrong. This is acting against the good of normal, healthy sex by impairing its nature.
Natural Law recognizes the good design of sexual intercourse. The Biblical perspective tells us Who was the Designer of this natural good, and for what purpose.
As I said, in the course of 1900 years of Christian reflection on God's purpose and design for sexuality, this has been a constant.
If people can ignore absolute Biblical and Christian consensus on that, lasting several millennia, it would not be surprising for people to approve other deviations from natural sex as well.
Maybe you've noticed.
??
And what is that?