THE MODERN ONAN'S WHO USE CONDOM AND SAFE SEX:

"And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. But Onan knew that the offspring would not be his; so whenever he slept with his brother's wife, he released his semen on the ground so that he would not produce offspring for his brother. What he did was evil in the LORD's sight, so He put him to death". (Gen 38:8-10)

Onan, was married to the widow of his brother, to preserve the name of his deceased brother that died childless. Onan seems to have been prompted to commit his crime by the motive of turning the whole inheritance to his own house. The custom of marrying the brother's widow was afterwards made one of the laws of Moses (Deu 25:5-6). This is called LEVIRATE MARRIAGE.

When brothers live on the same property and one of them dies without a son, the wife of the dead man may not marry a stranger outside the family. Her brother-in-law is to take her as his wife, have sexual relations with her, and perform the duty of a brother-in-law for her. The first son she bears will carry on the name of the dead brother, so his name will not be blotted out from Israel. (Deu 25:5-6)

Onan, though he consented to marry the widow, yet, to the abuse of his own body, of the wife that he had married, and of the memory of his brother that was gone, he refused to raise up seed unto his brother, as he was duty bound. 

What Onan DID was to begin the sex act with his dead brother's widow but to deny her his seed (semen is Latin for seed; sperm is Greek for seed) by pulling out and "destroying" it upon the ground.
Onan was put to death for POURING PUT HIS SPERM TO THE GROUND. 

Thus, God designed the semen or sperm for the only one destination -- the woman's vagina, womb, or sex organ. This means that the use of condoms, etc. that defeats this God-designed sex function is a sin and an abomination. YOU CAN'T CHEAT GOD AND GO FREE.