Lev 15:16-18 is right before my eyes and I do see it.
Many people disobeyed their fathers, disobeyed their customary obligations, and even disobeyed God Himself in the Bible and were not killed on the spot: this extraordinary penalty is clearly for the specific act that Onan did, which is mentioned in the very text: "[Onan] spilled his semen on the ground to keep from producing offspring for his brother. What he did was wicked in the LORD's sight" --- like I said, it is clear to every Biblical commentator up til the Age of Playboy.
The only reason Onan's seed spilling was a problem is due to the circumstances under which it happened. He was commanded to raise up children to his brother and rebelled.
Is killing someone a sin? Depends on when and why doesn't it. In wartime, not only is it not a sin but it is a good. Executing a criminal, (with the proper authority of the government) is a just thing, not a sin. Killing your neighbor out of spite however is a sin.
We have a similar situation here. According to LEV 15:16-18 the action he did is not sinful, but the reason he did it is.
Provan also recalls that the punishment prescribed for failure to "raise up seed for a dead brother" is not death, but merely to have the widow publicly remove her brother-in-law's sandal and spit in his face (Dt. 25:5-10).
Onan was the first example given of refusing to raise up children to his brother. The "spit in his face" thing was added to the law after this incident.
Provan's conclusion, based upon a close analysis of Leviticus 20, is that God forbids all forms of intentionally sterile intercourse.
Lev 20 covers all sorts of sexual liaisons. Most of which would not be intentionally sterile (but are unacceptable anyway becasue God says not to do these things). It does not cover intentionally not producing life at all.
Just like you can't find any Biblical argument in favor of deviant forms of intercourse --- until you get to the post-1950's American playboy culture.
No one said there was any support for deviant sexuality, although you do have to properly define deviant. (Almost anything a man and his wedded wife do, which is not explicitly condemned in the bible, does not defile the marriage bed.)
My point is that Onan was not killed for spilling his seed, he was killed for disobedience.
In fact, your argumnt in favor of deviant sexual intercourse
I would like you to take a deep breath and show me where I argued for deviant sexual behavior (unless perhaps your definition of deviant is extremely narrow). You will find that I did no such thing. I merely point out that Onan was not killed for spilling his seed as Lev 15:16-18 establishes that it is not sinful.
follows the same lines as the "Gay Christians'") argument in favor of gay sexual relations.
Actually not. relations between two men are explicitly prohibited in the bible, both old and new testaments. Apples and oranges
They, too, can answer every Scriptural rebuke with "Well, I doen't think there's any rebuke of homosexuality that applies to my own practice in particular --- that's not the way I read it."
And they do. But they are wrong. The bible is very explicit on the matter of relations between two men.
The bible is silent on non-reproductive sexual behavior between a man and his wife, however, except to say the bed is undefiled.
Not so. Certainly the interior dispositions or reasons why Onan committed the offensive thing he did included selfishness, disobedience, rebelliousness, greed --- but the thing he did was to engage in a deliberately sterile, "alternative" sex act; and the Bible says "what he did" was wicked in the Lord's eyes.
In every deed, we can distinguish between the objective nature of the act, and the subjective state of the mind and will of the actor. Moral condemnation comes from both the gravity of the evil act in itself, and the intentionality.
An honest look at the text shows that God condemned Onan for the specific action he performed, and not just for his anti-Levirate intentionality. An accurate interpretation of the text is that God found wicked what Onan actually did (asher asah).
This becomes even more clear in the context of the entire chapter, Genesis 38. Onan is only one of three of Tamar's male in-laws who violated the Levirate law. Onan's father Judah, and his younger brother Shelah, also violated the Levitate law, and Judah openly admits this in verse 26.
Of course, Tamar later tricked Judah into getting her pregnant; nevertheless his refusing to do this willingly is still a sin -- and yet neither Judah nor Shelah are punished by death for refusing to comply with the Levirate law. Only Onan is punished with death. And why is he singled out? Scripture tells us: because of what he did.
"Lev 20 covers all sorts of sexual liaisons. Most of which would not be intentionally sterile (but are unacceptable anyway because God says not to do these things). It does not cover intentionally not producing life at all."
"Intentionally not producing life" is not an accurate expression of the offense. Say there's a married couple that reasonably judges that it's not a good time to have a baby. They therefore abstain from sex during the wife's fertile phase (a week - 10 days per cycle.) They are "intentionally not producing life." But their intention not to produce life is not a sin.
On the other hand, if they accomplish this by choosing the kinds of sexual gratification that gay men choose (ejaculation of semen into the mouth and anus), there they are choosing acts which are objectively disordered.
Lev 15:16-18 does NOT establish that spilling the seed is not sinful. It establishes that even inadvertent "spilling" renders one unclean, though not sinful. The Onan incident reveals that doing it deliberately, IS sinful.
Notice this difference: the Hebrew verb used in the Onan incident (shachath) means something that is done intentionally, as to ruin or destroy; it is not the same as the verb used in Leviticus (shaphak), which lacks the emphasis of intentionality, and means merely to spill forth or slip. You can find this distinction in Strong's Hebrew Lexicon
You say that your methods of argument are not usable in a pro-gay sense, because "... relations between two men are explicitly prohibited in the bible, both old and new testaments. Apples and oranges."
Not so. Your methods of argument are routinely used by pro-gay advocates. These methods would include: (1) interpreting Scripture in an innovative way which suits your personal inclination, and (2) denying what Christian pastors have taught from Scripture for century after century. This requires that either (a) the Holy Spirit permitted each and all of these pastors to teach God's people wrongly for 2000+ years, or (b)the Holy Spirit taught some things in the early days, but started teaching something quite contradictory beginning around the mid-20th century.
I will illustrate from very typical "Gay "Christian" writings. Notice how gay advocates refute the oft-quoted passages which condemn the the kind of acts which homosexuals wish to engage in.
Most strikingly, there is no way to refute these arguments from the Biblical text alone. The gays will just say, "Well, I just go by what's in the Bible. And the explicit text of the Bible does not use the precise word of "gay" or "homosexual." It always refers to rapists, prostitutes, straight people who have abandoned their spouses in order to do something that's not natural for them, etc."
You and I don't agree with that interpretation. But you don't have any effective way to argue against it, because you think that the Church's interpretation of Scripture through the ages has no particular authority.