The following details answer the question wonderfully. To sum it up, yes, it is safe to swallow semen, if you know for sure it doesn't contain STDs.
1. Pregnant? Semen and sperm go down into your stomach, where they are digested with all your other food. There is no passage connecting your stomach to your ovaries. You can not get pregnant from swallowing sperm. If the donor has no STDs, it's perfectly safe. If you plan to have a baby with the man in question, it can also lower the chance of miscarriage. By having your body recognize the antigens in the man's semen you can reduce the possibility of an inappropriate immune response.
2. Fattening? Semen is full of protein, minerals (zinc Calcium), some fructose sugars and other nutrients. It has few calories, and is perfectly safe to swallow if it has no disease germs.
3. Your own? You can safely swallow your own semen. You can't catch a disease from yourself, only from other people. (Although you can spread your own germs to new, and perhaps more vulnerable, places on your body!)
4. Other people have diseases! You can catch gonorrhea, syphilis, herpes, chlamydia and genital warts from another person. All of these (except herpes and genital warts) can be cured with antibiotics, but it is annoying and inconvenient and unhealthy to get them.
HIV, the virus which can cause AIDS if untreated, is a far more serious thing. People die from AIDS. There is no cure, though daily medication for years can prevent it developing into anything dangerous. HIV virus is contained in body fluids, like semen, but it has to get into your blood through your bleeding gums or damaged teeth. If your mouth is strong and your gums are firm, then you can swallow the HIV virus without any trouble.
All these diseases (except HIV) are passed on through contact with sores or lesions, as well as in pre-ejaculate (pre-cum) and semen, so you may be exposed to them before your partner ejaculates. The safest thing to do with semen in your mouth is to swallow it. The enzymes and acids in your stomach will quickly destroy any viruses and bacteria. Or spit it out, and use a mouthwash to remove any lingering. Don't brush your teeth before or after oral sex. The brushing can cause slight bleeding of your gums leaving them open to infection from HIV.
If you swallow your own semen, then no, you can't get anything new. It's quite safe. It's fairly safe, too, to swallow someone else's semen.
no
Yes
I think it is :)
Yes. The amounts that might possibly be found in his semen are at such trace concentrations it won't do a thing.
Infectious diseases can be transmitted for semen so swallowing semen is not consistent with "safe sex". Other than that there is nothing particularly good or bad about it.
No. It's less risky than sharing a needle with someone HIV positive, but it's not "safe".
No. When you swallow the semen, it goes to stomach and digested. Sputum comes from your lungs.
No
Yes it is safe. I do not advise doing that though. It is safe and you can do so if you wish. You cannot give yourself any infection from taking your own semen. Many males masturbate and swallow their own semen. Recent studies have found that consuming human semen reduces depression. You can find an article about that in the related links below.
The rifampin in the semen will not cause any problems, but their semen may not be safe to ingest. Rifampin is an antibiotic given for many reasons, including STD's. If the rifampin is being given for an STD then it would not be safe to ingest the semen, not because it has the rifampin in it, but because it may still have infectious particles in it.
After all there is no harm in swallowing your partners semen.