The requirement of unity and indissolubility is directed in the first place against polygamy, which Christianity views as a fundamental offense against charity and human rights; it is also directed against what could be called "successive polygamy", a series of non-binding love affairs that never arrive at one, great, irrevocable commitment. The requirement of marital fidelity entails a willingness to enter a lifelong union, which excludes affairs outside the marriage. The requirement of open- ness to fertility means that the Christian married couple are willing to accept any children that God may send them. Couples who remain childless are called by God to become "fruitful" in some other way. A marriage in which one of these elements is excluded at the mar- riage ceremony is not valid. (YOUCAT question 262)