Sorry, but your logic contradicts itself.
It's a sin to have sex for pleasure alone. You have to have sex with the intent of bearing a child. Right?
If you try to play the odds, then you're attempting to have sex for pleasure while trying to avoid bearing a child. According to your own logic, that must also be a sin.
Get your head on straight! You're suggesting people be irresponsible about sex and bear more children than they can comfortably take care of, or even have children when they aren't ready. And if you pull the "God decides when they're ready", then why do you think it's okay for a man to have sex with his wife only when she's least fertile? If God decides when you have a child, then why does it matter if you wait until your wife is least fertile or most fertile before having sex? It's like you're trying to outsmart God.
Your logic is flawed. DO NOT PASS GO, DO NOT COLLECT $200.