Hi there.
I do think sex without intent to procreate is a sin, because it violates the reason sex was created. This is why I also disagree with the use of contraceptives, a position which unfortunately places me in agreement with the Catholics, which doesn't often happen!
Genesis 1:28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
Here we see the creation of man and woman, and God's instructions concerning sex - He didn't say to enjoy themselves & multiply, but just to multiply. As far as I can see, this is God saying sex is for procreation.
Now, if we look at Onan, I believe this tells us something about God's plan for sex:
Genesis 38:8-10 And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother. And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also.
Now, Onan was here commanded to follow the Law and give his brother's wife a child, which would be seen as his brother's child (Deuteronomy 25:5-6) but instead had sex, and practised coitus interruptus so that Tamar would not get pregnant by him. And God killed him for his sin. Now, yes, Onan had sinned by not following the Law's command and giving Tamar a child, but the penalty for that sin was not death:
Deuteronomy 25:7 And if the man like not to take his brother's wife, then let his brother's wife go up to the gate unto the elders, and say, My husband's brother refuseth to raise up unto his brother a name in Israel, he will not perform the duty of my husband's brother.
8 Then the elders of his city shall call him, and speak unto him: and if he stand to it, and say, I like not to take her;
9 Then shall his brother's wife come unto him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, So shall it be done unto that man that will not build up his brother's house.
10 And his name shall be called in Israel, The house of him that hath his shoe loosed.
As the penalty for refusing to give the wife of his brother a child would have been disgrace, not death, I don't see how we can say Onan was killed for not giving Tamar a child, as if God killed him for that, then God would be breaking the Law He gave, which doesn't make sense to me. As I see it, therefore, God killed Onan because he had sex without intent to procreate, because he used a contraceptive measure. This means contraception & sex without wanting to have children is a sin.
That is my understanding of what the Bible says on this, anyway.
/Lucilius