Genesi 38:9

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.

Ver. 9. When he went in unto his brother's wife.] God, for the respect he bears to his own institution of marriage, is pleased to bear with, cover, and not impute many frailties, follies, vanities, wickednesses that are found between man and wife. Howbeit, there is required of such a holy care and conscience, to preserve between themselves, by a conjugal chastity, the marriage bed undefiled; taking heed of an intemperate or intempestive use of it: which by divines, a both ancient and modern, is deemed no better than plain adultery before God.

Qui cum uxore sua, quasi eum aliena, concumbit, adulter est, saith that heathen. b Onan's sin here was self-pollution, aggravated much by his envy that moved him to it, expressed in these words, "lest he should give seed to his deceased brother." And the more sinful was this sin of his in spilling his seed; because it should have served for the propagation of the Messiah; therefore the Lord slew him: as also, because he was not warned by his brother's punishment. c

a Intemperans in coniugio, uxoris suae adulter est. - Aug. In uxorem alienam omnis amor turpis est, in suam vero nimius. - Hieron.

b Seneca.

c Hebraei inquiunt perinde ut homicidam, reum esse qui temere semen profundit. - Mercer., in loc.