Why did the Torah prohibit marriage/relations with Arayos/relatives?
Various reasons are offered in Mefarshim. Some explain relations with relatives were forbidden being that relatives are constantly found in one’s vicinity, and Hashem desires man to diminish his habit of sexual practice. If relations were permitted with relatives, man would often engage in intercourse due to the great availability. Other’s negate this explanation saying that the Torah permits one to marry even a 1000 wives, and hence clearly it was not trying to diminish man’s habit of sexual activity. On the contrary, from an intellectual perspective, there is no better option than to keep the inheritance in the family, and marry off one’s daughter to his elder son. In their opinion, there is no known reason behind the prohibition of Arayos, and it is a Chok, an unexplainable statute, of which its reason is only known to G-d. Alternatively, its prohibition is based on Kaballa, and how the mixture of seed of relatives causes confusion in the upper worlds. Other’s explain the Torah prohibited Arayos/relatives being that one is required to show respect to these relatives and it is improper to have a sexual relationship with them.
 Rambam in Moreh Nevuchim 3/49, brought in Chinuch Mitzvah 190; See also Even Ezra 18/6 “Since man’s inclination is like an animal’s, it is not possible to prohibit all women, and hence the verse prohibited only those women that are found with him constantly.”
 Ramban 18/6
 2nd reason in Rambam, brought in Chinuch ibid