From John Calvin's Commentary on Genesis, as found on BaylyBlog:
I will content myself with briefly mentioning this, as far as the sense of shame allows to discuss it. It is a horrible thing to pour out seed besides the intercourse of man and woman. Deliberately avoiding the intercourse, so that the seed drops on the ground, is doubly horrible. For this means that one quenches the hope of his family, and kills the son, which could be expected, before he is born. This wickedness is now as severely as is possible condemned by the Spirit, through Moses, that Onan, as it were, through a violent and untimely birth, tore away the seed of his brother out the womb, and as cruel as shamefully was thrown on the earth. Moreover he thus has, as much as was in his power, tried to destroy a part of the human race. When a woman in some way drives away the seed out the womb, through aids, then this is rightly seen as an unforgivable crime. Onan was guilty of a similar crime, by defiling the earth with his seed, so that Tamar would not receive a future inheritor.
In the comments, David Bayly, a pastor in the conservative Presbyterian Church in America, writes:
May I suggest that it's a frequent misunderstanding of our position to think that we believe all sex must lead to procreation or else be sinful. Tim is using Roman Catholic terminology when he speaks of "separating the unitive from the procreative" in sexual union. Roman Catholicism has thought more deeply and accurately about birth control than most Protestant churches and this is their way of saying that we sin when we seek to separate intimacy and pleasure as a result of sexual union from procreation as a result of union.
Thus, man does not have the right to say, "Today I am having sex for pleasure and intimacy, but not for children. In another year I will do it for children."
We must not separate the unitive from the procreative. We must not seek to preclude the gift of children from the act of intercourse--either by birth control prior to the act of intercourse or by abortion following conception.