Same sex relationship has recently been criticized by many not only on grounds of being immoral but also as a practice that erodes certain values attached to marriage and procreation in some African cultures. This paper examines the immorality or otherwise of homosexuality as an act. It argues that homosexual behaviour is degrading and damaging as an act because it devalues the institution of marriage and its related values in the Yoruba culture. The paper underscores the fact that a typical Yoruba would make allusions to either Christian or Islamic injunctions to justify the moral unacceptability of homosexuality as an act. It employs Aquinas natural law theory of morality to further argue that homosexual act is unnatural hence it goes against what reason dictates. The paper also argues that we cannot completely rule out the possibility that homosexual acts are consequences of certain predisposing biological factors over which actors have no control and if this is the case, then they cannot act otherwise. In the light of this, the paper argued for both nature and nurture as predisposing factors of homosexual behavior. It concludes by stressing that in spite of some biological arguments in favour of homosexual act, such act threatens the viability of the Yoruba community where special value is placed on the institution of marriage and procreation.