Is the primary purpose of marriage to have children, or to share love between the spouses?

Everything you conclude will be based on your fundamental definition, so we have to get it right. Is marriage for sharing love between two people? The answer is no. Love is certainly part of marriage; without it a marriage would not last two weeks. But there is one thing marriage produces that no other relationship produces, and that is children. Pius XI settled this in Casti Connubii, paragraph 17: the primary end of marriage is the procreation and education of children. He does not add that love is also a primary end; he says there is only one primary reason. Later, in paragraph 59, he lists the secondary ends: mutual aid, the cultivating of mutual love, and the quieting of concupiscence, which husband and wife are not forbidden to consider so long as they remain subordinate to the primary end and the intrinsic nature of the act is preserved. Humanae Vitae, paragraph 12, confirms this with the inseparable connection between the unitive and the procreative meanings of the marriage act, without disagreeing that children are primary. It is like eating: you eat what you enjoy, but the primary reason for eating is nourishment, and enjoyment is secondary. So with sex, the primary reason is to have children, and marital love and the rest are secondary. That is where Trent Horn goes too far toward the unitive at the expense of the procreative.