If by onanism you mean masturbation, then you are wrong. Onan's sin was not that he spilled his seed but that he disobeyed God by not raising up children to his brother. (He could have spilled all the seed he wanted AFTER he impregnated his brothers widow. )
Here is a discussion from another site on the issue..
(http://www.dtl.org/ethics/e-mails/masturbation/follow-ups.htm)
Masturbation is not mentioned by name (maybe they didn't have a word for it), but I did find a part of the Mosaic Law that applies to it. Bear with me for a few moments.
Leviticus 15:16-18 reads:
If any man has an emission of semen, then he shall wash all his body in water, and be unclean until evening. And any garment and any leather on which there is semen, it shall be washed with water, and be unclean until the evening. Also, when a woman lies with a man, and [there is] an emission of semen, they shall bathe in water, and be unclean until evening (NKJV).
This is normally explained as being a "nocturnal emission" but if you look under the hood at the Hebrew, a different picture evolves. The word "emission" is alah, which means (among other things), "(cause to, make to) come (up)." The word translated "semen" is a compound of shkabah ("a laying down for") and zeroa ("something sown"..."seed"). So, this word choice seems to indicate a deliberate action of producing the seed.
I'm afraid the "passive" sound of the verses in question is a result of religious sensibilities and not the most honest possible translation of the underlying language. The woman's presence is obviously optional (not present in verses 15 or 16) but present in 17. So I put it to you that masturbation is definitely implied by the verses. Even sticking with just the English translation, were it a nocturnal emission, would the phrase "be unclean until evening" make a lot of sense?
Let me take this just a touch farther. In chapter 15, there are two situations that require sin offerings and one that doesn't. This emission from semen is the one that does not require a sin offering. They do become ceremonially unclean, and they do need to wash up afterwards, but that's the beginning and ending of the requirement under the Law. If there is no sin offering required, then no sin has been committed.
Note that any sexual thoughts that may accompany the act are still sinful. (Lust is condemned in the bible)
Not so. Violation of the Levirate law was not a capital offense. If a man didn't fulfill his obligations to his deceased brother's widow, she was to "go up to him and strip his sandal from his foot and spit in his face, saying publicly, 'This is how one should be treated who will not build up his brother's family!'" (Deut. 25:9).
Yet Onan received, not a humiliating public rebuke, but a death sentence. Why? Because he sinned not only by violating the Levirate law, but also by the way in which he did so. The kind of act he committed was so despicable that, in the Old Testament context, it was punishable by death.
Onan went through the motions of the life-giving act --- a kind of parody of real sex --- but deliberately altered it so that natural fertility was evaded by an unnatural act, one with no life-giving significance.
Incidentally, Protestants like Luther, Calvin, Zwingli, Wesley, Melancthon Jacobus, Matthew Henry and many others saw the act of "wasting the seed" as being morally abhorrent in the eyes of God, as you can see at the link.
Lust itself, as you point out, is condemned in the Bible, but I'm sure you will agree that this is not a condemnation of sexual desire per se. The Song of Solomon is clearly a celebration of the sexual desire of the betrothed lovers for each other, and the New Testament upholds the "honor" of the marriage bed, even going so far as to say that the husband's cherishing of his wife's flesh, and his care for her, is a "Mysterium Tremendum" which is an image of Christ's love for the Church.
I think you'd have to say that a desire that leads to blessed natural marital union is approved by God; while on the other hand the desire that leads to defective acts, whether by yourelf, with your spouse, or with somebody else, is not blessed, and is even offensive in the eyes of God.