So says you, and I really see no reason at all to believe you over how this passage has been interpretted for centuries. This reminds me of how some Protestants also claim that the Bible doesn't really condemn homosexuality.
Non sequitur, and attempted diversion.
You are WRONG when you say that Genesis 38 has been interpreted as such "for centuries". You would be RIGHT if you had said, "SOME PEOPLE have interpreted it as such for centuries."
Was Onan the only person to EVER "spill his sperm on the ground"? If not, why isn't everybody slain for that very thing? 
God has spoken my friend.
Yes, God HAS spoken.
Unfortunately for you, God has NOT spoken what YOU are speaking.
YOU are claiming to speak for God where He simply has not spoken. I would be VERY nervous about doing that!
Rarely the case. Even when this is the case, trusting God is the nobler path. My wife's godmother was told another child would kill her two children ago.
Another non sequitur.
It would be nothing short of STUPID for my wife & me to have another child at this point in our lives. You are simply too young, too naive, and too arrogant to understand that.
But you will some day...
No. Chances are still small, and children with birth-defects are still blessings.
Yet another sanctimonious non sequitur.
You mean people like Americans who can afford cars, computers, internet access, and so on. People who are in the richest 20% of the world's population, with food so abundant that even the poorest are fat.
Wrong again.
And this is completely and totally irrelevant anyway.
Talk to me in 20 years, when you have 25 kids and your grocery bill is running you about $3,000 per month.
I haven't been in anyone's bed, and I wouldn't have even known your own practices if you hadn't felt that it was necessary to tell me.
Yes, you ARE getting into married couple's beds and telling them what they can and cannot do!