The Necessity of Swallowing - The Sin of Spilling Seed
Most of us are familiar with the Biblical story of Onan, whose sin against God was that of spilling his seed on the ground:

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. 10 And the thing which he did displeased the LORD: wherefore he slew him also. (Genesis 38:9)
This scriptural passage has traditionally been used as an injunction against masturbation. However, upon closer reading, it becomes apparent that this scenario has nothing to do with masturbation at all. Onan was not masturbating; he was copulating with his brother?s wife (and there was a good reason for that, in God?s plan). His sin was pulling out (coitus interruptus) and ejaculating on the ground rather than into the woman. He did so in order to avoid impregnating her. However, he could have easily avoided God?s wrath (and the penalty of death), by simply having the woman fellate him and then swallow his semen. This would have kept him from impregnating her, as well as completely prevented the spilling of seed that was an offense in God?s eyes.

The extreme case of Onan aside, how bad is it in general to spill semen? The Old Testament ranks it with other acts of uncleanliness that meet with God?s disapproval: And if any man's seed of copulation go out from him, then he shall wash all his flesh in water, and be unclean until the even. 17 And every garment, and every skin, whereon is the seed of copulation, shall be washed with water, and be unclean until the even. (Leviticus 15:16-17) Getting ejaculate on oneself or one?s clothing results in uncleanness that requires extensive reparations and atonement. Obviously one simple way to prevent the spillage of semen is to have your partner perform fellatio and swallow the emission. In fact, in light of these scriptures, performing fellatio to completion and then spitting out the resulting emission seems almost unthinkable.