Although many people in today's magical community find it a bit off-putting, the use of bodily fluids in magic is a long-standing custom in many cultures and traditions. Even if we think of it as unpleasant, it's disingenuous to pretend that no one has ever used -- or may presently be using -- things like blood, semen, or urine in their magical practices. In many forms of magic, bodily fluids are considered a bonding agent. This make them the perfect taglock, or magical link. Blood, in particular, is seen to be particularly powerful, for a variety of reasons.
In hoodoo and some folk magic customs, a woman's menstrual blood is considered vital to some types of magic. Jim Haskins says in his book Voodoo and Hoodoo that "to keep a man crazy about her and uninterested in wandering, a woman simply has to mix some of her menstrual blood into his food or drink."
A North Carolina folk magic practitioner who asked to be identified as Mechon says that growing up, the men in her family knew not to eat any foods that might have a woman's blood hidden inside it. "My uncle would never eat spaghetti, or anything with tomato sauce," she says. "The only way him and his brothers would eat things like that was if they were at a restaurant. They knew the women could control them with the blood if they ate it."
In ancient Greece and Rome, blood was considered to have strong magical properties as well. Capitolinus writes of the empress Faustina, wife of Marcus Aurelius. Faustina once was consumed by her lust for a gladiator, and she suffered greatly over this. Finally, she confessed to her husband, who discussed the matter with the Chaldeans oracles. Their advice was to order the gladiator killed, and have Faustina bathe herself in his blood. While covered in it, she was to sleep with her husband. Faustina did as she was told, and she was "delivered of her love for the gladiator." She also happened to be delivered of a son a short time later, Commodus, who was quite fond of gladiatorial games.
Pliny the Elder relates the story of the mage Osthanes, who used blood from a tick found on a black bull to control a woman who might be unfaithful to her husband. He says, "If the loins of a woman are smeared [with the blood], she will be made to find sex repulsive."
In parts of the Ozarks, there is a belief that dried blood on a floor will liquefy as a harbinger of destructive storms to come.
Urine is sometimes used in magic as well. Historically, one might have placed urine in a witch bottle, as protection against harmful magic and sorcery. However, Haskins explains that it can be incorporated into a curse as well. He says to obtain some of the intended victim's urine and put it in a bottle. A few more ingredients are added, the bottle is buried and stomped on, and the target will die of dehydration. On a slightly less malevolent note, he also says that mixing the urine of a young girl with saltpeter and then drinking it as a tonic will help restore a man's "lost nature," if his woman has used magic to command sexual loyalty.
Havelock Ellis says in Studies in the Psychology of Sex that urine was sometimes sprinkled on newly married couples, as a blessing - a bit like holy water. The Greeks often mixed urine with salt, and then used it to asperge a sacred space.
In some magical traditions, semen and vaginal secretions are an important component of sex magic. Cat Yronwoode recommends the gathering of semen in a discarded condom, and points out that it can easily be frozen until such time as it is needed. Folklorist Harry Middleton Hyatt documented cases in which a man's "nature" -- or his wandering eye -- could be "tied up" in a napkin, which would keep him sexually bound to one woman.
So, in this day and age of highly communicable diseases, should you be using bodily fluids in your magical workings? Well, like many other things, it depends. If you're using your own fluids in a working, and you're the only one who's going to come into contact with them, then it should be fine. If you're using someone else's bodily fluids, or using yours with the intent of sharing them with another person, you may want to exercise a bit more caution. Safety is paramount.
If you are unable to obtain bodily fluids -- or if the very idea makes you cringe -- there are plenty of other options available. Be sure to read about taglocks and magical links for some suggestions on other items to use in workings.