The following excerpt from Marriage and love in England : modes of reproduction, 1300-1840, by Alan Macfarlane, describes how romantic love leads to disintegration of the extended family and promotes lowered fertility among those who practice it. 

Romantic Love

In contrast to most other recorded societies, it has been noted that Americans do ‘not merely build their households on the husband–wife relationship, but build their whole value system and morality on it’. Whereas in industrial Western societies the emotional relationship between man and wife is primary, it is not the pivot of social structure in the majority of societies.¹ As we have noted, the relationships that are most important are often those between parents and children, with the marital bond as a poor second. ‘In Eastern countries with their ancient civilization there exists even now comparatively little of that tenderness towards the woman which is the principal charm of our own family life,’ wrote Westermarck.²

The transition from a situation where the marital relationship is subordinated to others, to the prevalent Western view of it as the deepest and most enduring relationship of life, brings numerous consequences, changing the nature of marriage and women’s and men’s roles. One consequence is the demographic one. The substitution of the husband–wife relationship for wider kinship links and parent–child ties, decreases the pressure to have children. The couple are sufficient to each other: children become a luxury, not a necessity. Effective contraception makes it possible to choose whether to indulge in a few or many. Hence the strengthening of the husband–wife bond is part of that emotional and economic nucleation which certain demographers like Caldwell see as leading to a state of affairs propitious for the demographic transition to lowered fertility.³ 

¹ Bohannan, Social Anthropology, 99; Goode, World Revolution, 14, 89.
² Westermarck, Marriage, ii, 24ff; ii, 28.
³ Caldwell, ‘Restatement’, 354.