Swallowing semen from a disease-free male during sex is generally safe. There is nothing unhealthy about swallowing unless the person has an STD. Bodily fluids except for tears and saliva are agents that transmit some STD's such as AIDS and HIV.
http://www.dontspitswallow.com/cum_safety.shtml (exact quote)
Many people have questions about oral sex and disease transmission: Can you catch an STD from oral sex? Can get other diseases from swallowing cum? Will swallowing cum hurt you or make you sick? Is it safer to spit than to swallow? We'll attempt to demystify some of these issues.
First the Good News Cum is an all-natural bodily fluid that doesn't harbor any diseases or harmful bacteria in a healthy person. The purpose of semen is to transport sperm to the female ova, and it is a high-protein, nutrient-rich medium designed to support and sustain the spermatazoa during their relatively brief journey. The sperm themselves also present no threat in the digestive tract, and are neutralized by gastric fluids by the time they reach the stomach.
If anything, swallowing the cum of a healthy male may in fact be good for you, since it is a highly concentrated source of protein, minerals, natural sugars, and other nutrients. Healthy seminal fluid is mostly fructose and proteins, and doesn't contain any harmful chemicals. The slightly bleachy smell of semen comes from its alkaline pH and minute traces of chlorine, but if it's mild enough for a vagina, it's not going to be any harsher on your mouth or throat.
Because the ducts and glands that produce the components of semen feed into the urethra, the same passage that carries urine out of the penis, cum may pick up some traces of urea or uric acid. However, these are harmless, as is healthy urine in general. The urinary tract of a healthy person contains only trace amounts of bacteria, and there are no substances in urine that are harmful to consume in the quantities excreted normally.
The Not-so Good News The sort of good news is that swallowing your partner's cum will not put you at any greater risk for catching an STD. The flip side is that if your partner does have an STD, by the time you've had unprotected oral sex with them, whether you swallow or not, you've been exposed to whatever they're carrying.
Having unprotected oral sex (giving a blow job without a condom) is risky. Some of the disease that can be transmitted through oral sex if your partner is infected include herpes, genital warts, gonorrhea, chlamydia, and syphilis. The risk of HIV transmission through oral sex is still undetermined. Some experts say there is little to no risk of getting HIV through fellatio, while some say there is possible risk, especially if there are cuts or sores in the mouth of the person performing oral sex.
Because most of the STDs mentioned are transmitted through contact with sores or lesions on the genitals, or are present in the pre-ejaculate fluid (precum), a person performing unprotected oral sex on an infected partner has already been exposed even before their partner ejaculates. If your partner cums in your mouth, you may be exposed to additional quantities of STD pathogens in the semen, which can infect you through the mucous membranes in your mouth and throat or through small cuts or scrapes on the inside of the mouth.
However, if you've already gone that far, swallowing the semen isn't going to put you at much additional risk. If anything, once the cum is in your mouth, swallowing may be the quickest way to get rid of it. Once you've swallowed, your digestive enzymes and acids will probably destroy any germs in the semen, and your stomach and intestines are not vulnerable to infection by STDs.
If your partner has cum in your mouth and you are concerned about STDs, you can minimize your risk by either quickly spitting or swallowing. The point is just to get the cum out of your mouth quickly one way or another. Then whether you've spit or swallowed, gargle and rinse your mouth thoroughly with a 3% hydrogen peroxide solution (the type available in drug stores for first aid). This will kill any germs in your mouth and throat and wash them out of cuts in the mucous membranes.
But It Made Me Barf Although there is nothing in healthy semen that can cause illness, some people get nauseated or even vomit during or after performing fellatio. This is a result of the gag reflex being triggered and is not a cause for alarm. The gag reflex is a way that your body protects you from choking, and is activated when something - like the head of a cock -- touches the back of the throat, stimulating certain nerves. If the reflex is stimulated enough, it may cause involuntary vomiting. The gag reflex can be controlled by practicing relaxation techniques. If this is a persistent problem, you may want to try a different angle when performing fellatio.
Other people may become nauseated after swallowing semen for psychological reasons, or because they find the taste or consistency of the semen unpalatable. If you find oral sex or semen repugnant, addressing underlying psychological causes may be necessary. If the taste or consistency of the semen itself is objectionable, there may be dietary changes that your partner can make to improve the flavor of his spunk.
The Bottom Line Unprotected oral sex, whether you swallow or not, is risky if you don't know your partner's health status. Oral sex without a condom, whether or not your partner cums in your mouth, can transmit several STDs, if your partner is infected. However, swallowing cum will never, in itself, give you an STD or any other disease, and swallowing does not significantly increase your chances of contracting an STD.
If your partner carries any viruses or diseases then yes he could transmit it to you, including HIV although its not a very high probablility of transmitting it this way. There are many viruses that can be transmitted, but the sperm itself, if clean will not have any bad effects on your health. It has no harmful effects at all.
It is fine to swollen semen and it's impossible to get pregnant that way.
Yes it is safe providing the man isn't infected with a STD.