First off, if semen dry outside of the body then they are, pretty much, dead. Sperm need a warm, moist environment to survive in. This being said, once the sperm enter the vagina, they have live for DAYS with no problem. The vagina is one of the best environments they can be in.
Secondly, sperm can be killed off inside the body if the male currently is receiving an injection to make him infertile. Apparently the government is in the process of making this "vaccine" though I do not believe it has been released to the public. Basically it would make the male discontinue producing sperm and the body could never start producing them again, leading to complete infertility. Sometimes men can become sterile from childhood trauma but only a doctor can truly make that determination.
Here is a link explaining some of the "vaccine" which prevents fertility:
Next, soap is effective on killing sperm. This being said, many men and women like to wash their hands thoroughly after mutual masturbation so they do not worry about sperm entering the vagina from moist fingers.
There is also a theory that if you have a more acidic body that the Y sperm (for making males) will be killed off by the different PH balance inside the vagina. This decreases chances of having a male offspring; however, it does not decrease the chances of pregnancy. Y sperm are naturally more fragile than X sperm and are killed off easier than X sperm. Doctors claim that if you are trying to conceive a boy then to stay away from acidic foods like tomatoes and oranges so that more Y sperm live too.
This being said, some sperm may have been neutralized by natural antibodies or hostile cervical fluids. Again, this is not the entire ejaculate being neutralized.
Spermicide may also kill off sperm; however, it is no effective on killing off all of the active sperm. You can find spermicide on some condoms and can purchase it in vaginal foams and such for sexual activities.
In addition, gastric fluids neutralize sperm so it is safe to ingest. The acidity in your stomach would not allow sperm to survive; however, saliva will not kill sperm and is unlikely to damage semen.
In short, yes, sperm can be neutralized; however, it is a little more difficult to figure out once it's actually inside the vagina.
venom_97 answered Wednesday June 18 2008, 4:03 pm: Yes, sperm can be neutralized in 3 different ways that I know of. cervical fluids, antibodies, and certain foods can cause it too. [ venom_97's advice column | Ask venom_97 A Question ]
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