Because they’re madly in love with their spouses and they (… um … how shall I put this?…) like to express it! (If you know what I mean.) 😉
And they don’t use birth control. (Gasp!)
See Pope Paul VI’s masterful work, Humanae Vitae, for details.
Members of Opus Dei are encouraged to be exemplary in their witness to the beauty of large families. In today’s world where so many people, Catholics included, view both children and the elderly as distasteful burdens, this witness is sorely needed. Still, members must exercise their own judgment about the number of children to have. If, in their own conscience, formed in the privacy of their own prayer they believe they have a serious reason to limit the number of children, they enjoy the same freedom other Catholics have to use Natural Family Planning. It’s good for all Catholics to consult a priest in this matter when making this decision, but the decision belongs to the couple alone. The decision should be made taking into account the financial, physical, and emotional needs of the spouses and the children they already have. This includes the likelihood of being able to provide a suitable home and an adequate education.
Over the years I have seen slanders which claim that Opus Dei insinuates itself into this and other intimate decisions between spouses. This has never been my experience or the experience of others in Opus Dei to whom I have spoken. I would never have tolerated such an intrusion and I don’t know of anyone in the Work who would have.