KJV Gen 38:8-9 reads:
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
into under his brother's wife, that he spilled it on the ground, lest that he
should give seed to his brother.
The 1867 RLDS Inspired Version reads:
And Judah said unto Onan, Go and
marry thy brother's wife, and raise up seed unto thy brother. And Onan knew
that the seed should not be his; and it came to pass, when he married his
brother's wife, that he would not lie with her, lest he should raise up seed
unto his brother.
The JST makes it clear that Onan married the woman first and then was
expected to engage in sexual intercourse; the KJV lends itself to coitus being
expected before the marriage with the euphemistic command to "go in unto thy brother's wife" preceding "and marry her."
In Old Testament Manuscript 2, pp. 61-62, we read:
8th
and Judah said unto Onan, go and marry thy brothers wife and raise up seed unt[y]<o>
thy Brother <9th> and Onon knew <that> the seed should not be his; and it came to pass, when
he married his Brothers wife, that he
would not lie with her, Least he should raise up seed unto his Brother