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The rythym method and Onanism are two different things. Onan was killed for withdrawing. The rythym method involves abstaining from intercourse on days when the woman is fertile. There is nothing wrong with the latter practice.

I honestly can't imagine why withdrawl is wrong but the rythym method is OK. Please explain.

There was a little more to the story than mere withdrawal.

Onan's elder brother had died (slain by God, for being "wicked in his sight"). Onan's father Judah commanded his son to marry his brother's widow, Tamar, and impregnate her so as to give his dead brother a "descendant". In other words, by custom, the child would be considered the child of his dead brother, not him.

Onan apparently resented the fact that the child would be considered his brother's. That did not stop him from gratifying himself sexually with his brother's widow--but he deliberately witheld from her the child she had been promised (which was probably the only reason she was in his bed in the first place, so you could say he deceived and defrauded her). And in doing so, disobeyed his father's explicit command, and the custom of the time. So God killed Onan too.

Judah told Tamar to go back to her father's house and wait for his youngest son to be old enough to marry, so as to give her a child to raise as her dead husband's. But he broke that promise and the marriage was not performed. Tamar finally got herself pregnant with a child of her husband's blood (twins, actually) by disguising herself as a prostitute and seducing her father-in-law Judah.

So I don't know if the act of withdrawal would have warranted death under different circumstances, but Onan was selfish, spiteful, derelict in his duty, defiant of his father, and just plain evil to poor Tamar. I'd say he deserved a good smiting.

I honestly can't imagine why withdrawl is wrong but the rythym method is OK. Please explain.

Gladly. Sexual intercourse is a divinly ordained act whose purpose is twofold: procreation and expression of love between man and wife. These two purposes are intimately joined by God, so it is a sin for man to separate them. Hence all methods of contraception, including Onanism, are wrong, as are all methods of sterilization.

A married couple, however, is not obliged to have intercourse every day. In fact occasional abstience is encouraged by St. Paul. Hence it cannot be sinful to abstain from intercourse during the woman's fertile days. In so doing the couple does not separate the two ends of the sexual act; it is God, not man, who made it so the woman cannot be impregnated on certain days of the month.

Of course, it is wrong for a married couple to avoid having children altogether. In fact, all married couples are called to be generously fruitful according to their means, which of course vary from couple to couple. Use of the rythym method is in harmony with the divine law so long as the couple fulfills this obligation.

For more on this subject, I suggest you read Humanae Vitae.