There is no law that prohibits dating between first cousins. In fact, there is no law that prohibits a relationship with absolutely anyone you choose, given they're of the legal age of consent. However, should you consider marriage with a first cousin, in most countries, the marriage of first cousins is prohibited.

Having sex with a cousin is a very personal decision, although, as you already know, people will always have their own opinions and make their own comments. Most often people are apprehensive about first cousins, or family relatives who fall "in love" because there are very often negative biological consequences in the offspring of blood relations. The ramifications of offspring from second cousins or more distant relatives are minimally greater than for offspring of non-relatives, but for those more closely related than second cousins, there are tremendous disadvantages to their consanguineous consummation.

Physically or mentally impaired children are frequently born of blood relatives because previously hidden recessive genes are more likely to appear. The closer the parents are in blood relation, the higher the chances are that they share similar genes. "Hidden genes" may contain beneficial traits, but they are more likely to contain detrimental traits because recessive gene mutations are most often a result of their inefficiency. Studies of first cousin offspring show that there is a 6 to 8 percent chance that the child will have a birth defect, in contrast to the 3 to 4 percent rate of birth defects from children born of non-relatives. In other words, the descendants of first cousins are almost two times more likely to suffer an abnormality or death than children of the average couple.