Onan's smiting by God for "spilling his seed," the command to "be fruitful and multiply," the statement that children are like arrows, and "happy is the man whose quiver is full of them," condemnations in the Pauline corpus of sexual behaviors that exclude the unitive and procreative powers of physical intimacy.
Onan was not smitten for that per se, but for disobeying Joshua. Read Genesis 38:8-10 for the context.
the command to "be fruitful and multiply,"
Hmm. That was written in stone, was it? It seems to me that He was just talking to Noah, and not the whole human race.
The rest I agree on--children are a blessing, and a man can be very happy with children, as I am. But lack of blessings does not sin make.
Paul's condemnation of sexual behaviors did not reach into legal marital, normal sex. By that, I mean loving sex between man and wife.