When I debate pro-aborts, I often hear some version of this argument: "But aren’t sperm and eggs alive? Does masturbation kill children? Or are there children on used tampons?" It’s a really bad argument, but I hear it a lot. The idea seems to be to ridicule Christians and try to throw out life at conception arguments by pretending that sperm are people too (or at least that Christians think so).
The root of this idea, that Christians think it is murder to kill a sperm cell, seems to come from a passage in Genesis 38:8-10 where a man named Onan is killed by God after engaging in coitus interruptus.
King James Version
“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 in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother. And the thing which he did displeased the LORD: wherefore he slew him also.”
The argument that Onan's sin was ejaculating outside a woman is a Catholic interpretation that is not consistent with the rest of the Biblical text or an understanding of Bible times. Furthermore, this is one of the ideas that has put ammunition in the hands of pro-choicers to claim that Christians think sperm are human beings and shouldn't be "killed” – either in an attempt to ridicule Christian beliefs or to argue that life doesn’t begin at fertilization.
In interpreting this passage, it’s important to look at the context, the rest of the Bible, and also to have an idea of the culture at the time. Onan's actual sin was marrying a woman (Tamar) and then flat out refusing to provide for her. The problem wasn't that he "spilled his seed on the ground," but that he didn't ever plan to get her pregnant at all. The text even says that was his motivation.
Onan only married Tamar, his dead brother's wife, because it was his duty to provide an heir for his brother and a child to care for Tamar in her old age. Refusing to give her a child meant that Onan would inherit more (his brother would have no heir to get a share of the family inheritance), so he planned never to get her pregnant, even though that would mean Tamar would be left alone with no child one day.
Other translations make this even more clear.
English Standard Version
"But Onan knew that the offspring would not be his. So whenever he went in to his brother’s wife he would waste the semen on the ground, so as not to give offspring to his brother."
Note that it says "whenever" he had sex with Tamar, he spilled the semen on the ground. This indicates, not just one contraceptive act, but a pattern intended to deprive her and his dead brother of an heir. The wording in the King James version is vague as to whether it was one time or a pattern. It could be either, although the phrase "and it came to pass" usually indicates the passage of a period of time and thus suggests multiple instances. Most translations, however, use wording that indicates an on-going pattern of refusal to produce a child. Thus, the sin of Onan is made more clear in being a refusal to produce a child out of selfishness, not a one time act of placing sperm outside a woman's vagina.
Also, given that the sin of Onan was serious enough to cause God to strike him dead, one would think it would be clearly spelled out elsewhere in Scripture. If the sin was putting sperm somewhere besides a woman's vagina, there is no mention anywhere else in Scripture that this is forbidden. On the other hand, if the sin was refusing to provide a child to his wife and thus to fail to provide for her old age as well, there is other Scripture that is consistent with this. For example, the Bible tells us in I Timothy 5:8 that a man who fails to provide for his family is “worse than an infidel” (i.e. an unbeliever). So, apparently, God does consider it a grave sin for a man to fail to provide for His family.
So the Bible does not teach that sperm are sacred or that they can only be placed inside a woman. And it certainly does not teach that they are human beings.
My position makes much more sense of the Bible as a whole, I think, but it also makes sense of the science involved in human reproduction in which sperm die naturally every day. Sperm die and are reabsorbed in the male reproductive tracts if they aren't ejaculated. Sometimes sperm are released during nocturnal emissions (i.e. "wet dreams"), which is perfectly natural. Of the millions of sperm that are ejaculated at any one time, the vast majority never even reach the egg (assuming there is even an egg there), and of those who do, only one (at most) will actually fertilize it. Human biology sure doesn’t seem designed to keep sperm from dying. The design of the human reproductive system doesn't seem to indicate any principle that sperm are anything more than just cells or that the death of a sperm cell is cause for alarm or that sperm must be conserved and only released inside a woman.
Most importantly, we know from science that only after a sperm and egg fuse is there a separate human being. Sperm and eggs by themselves are just cells - no different than a skin cell or blood cell in that regard. They're part of the body they came from, but broken off, as it were. They aren't separate living organisms.
Of course, there are Christians who disagree with me on the topic of contraception and do think it is a sin. However, even these people agree that contraception does not kill a human being. Nobody is claiming that sperm are human children.
Killing sperm is not the same thing as killing a human individual. That's biological fact, regardless of anyone's position on contraception. Sperm are not human beings.