As evidenced by the mockery of the Lucky Mojo Forum a while back, the magical method of trapping a lover by feeding (usually) menstrual blood, or sometimes urine, to the target, is one of the more uncouth practices used in folk magic -- many nonbelievers will laugh and shake off mention of dusting items with herbs or burning fixed candles as harmless, but the act of secretly feeding human bodily secretions to another person tends to cross the line into being repulsive (not to mention, frankly, illegal in many places if it's done without consent.)
According to the page at Lucky Mojo about bodily fluids, "In the African-American hoodoo tradition, as well as in Sicilian folk-magic, menstrual blood served to a man in his coffee or tea is a sovereign recipe for capturing his sexual attention. No ritual, prayer, or invocation is necessary; you simply add some menstrual blood to the man's coffee or tea. The idea is to get your scent into the beloved's sphere of consciousness ... In European -- especially Italian -- folk magic, as well as in hoodoo, urine is used in women's coffee and tea love spells, as a quick substitute for menstrual blood, when the intention is to tie or bind a lover. This is particularly common among girls to young or women too old to have menstrual periods, and among pregnant women and those who for any other reason don;t (sic) menstruate. Men also can put urine in a drink to tie a woman, although this is not mentioned so commonly."
My first contact with this tradition actually came via Snopes.com where their article on menstrual superstition reports, "(Menstrual blood has been) viewed as a potent ingredient to add to a love potion or to slip directly into something that would be ingested by the lusted-for object of affection. A drop or two secreted in a man's food or drink was said to bind him forever to the woman whose blood it was. (Didn't work, though: history is replete with disappointed maidens who attempted to win themselves adoring husbands through this trick.)"
The trick is probably popular because it's cheap (practically free!), doesn't require any odd ingredients, and the only task of any difficulty is in the actual collecting/deploying the fluids. Plus, as the spell is usually given, it's promised to bind the man to you forever! -- which is usually what people are striving to do. Though yronwode claims on the page linked earlier to have always had good results with this trick, I'm inclined more to agree with Snopes about its effectiveness -- I've met a few people that have used this or been the victims of its use, yet none of these people seem to still be in the relationship they were supposedly obliged to stay in. Most of them didn't use any fancy uncrossing spells or escape spells either; the thing just plain didn't work.
Could it have been the squeamishness of the deployer creating a negative effect on the magical powers? Except in one case of a friend who'd had this done, she was already broken up with the guy before realizing he'd put something on her earlier, indicating it still won't stop a person from leaving. A friend of mine from a Cuban/Santeria background has said that as she was taught, the menstrual blood does produce strong love but only for very a short time, and so these accounts match with what she was told.
Just my opinion of course, based on what I've understood -- but I'd say this is one of the tricks not worth getting involved with. It seems to just annoy the targets and not work very well, and if done without consent can land you in jail.