If the donor has no STDs, it is 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 mans semen you can reduce the possibility of an inappropriate immune response.
In case the person is infected, you can catch diseases like gonorrhea, syphilis, herpes and chlamydia. 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.