Can 1095 The following are incapable of contracting marriage:
2/ those who suffer from a grave defect of discretion of judgment concerning the essential matrimonial rights and duties mutually to be handed over and accepted;
Can. 1096 §1. For matrimonial consent to exist, the contracting parties must be at least not ignorant that marriage is a permanent partnership between a man and a woman ordered to the procreation of offspring by means of some sexual cooperation.
§2. This ignorance is not presumed after puberty
Can 1101
§2. If, however, either or both of the parties by a positive act of the will exclude marriage itself, some essential element of marriage, or some essential property of marriage, the party contracts invalidly.
vatican.va/archive/ENG1104/__P40.HTM
Those seem to be the relevant canons. I think the SSPX is basing their case on the first two. I, on the other hand, was thinking of the last citation, Can 1101.2. (And, just to be clear, I was thinking muddily when I said the vast majority are invalid on grounds of openness to life because I was lumping together two cases - a) those who intend to never have children and b) those who intend to contracept but who I highly doubt actually believe marriage is ordered toward procreation; the cases are different.)
Taking those who fully intend from the beginning never to have children, I think this falls under canon 1101. They are willfully excluding the end of marriage from their vows. The end of marriage is an essential component thereof, so it seems excluding it would render the marriage invalid.
In the case of those who merely intend to contracept, however, I think the SSPX has a case from the canons while I would also contend that the presumption in this day and age that a couple knows what marriage is turns out to be rather ludicrous. Will a priest
possibly mention that procreation and education of children is integral to marriage? Sure, so they would have heard it commented on once in their lives. But can we really presume that they understand the centrality of this to the Church’s teaching on marriage? In today’s world I highly doubt it.
In the end, I’ll stand by my assertion for those who intend 0 children but I’ll give you a touchee for the presumption that couples know what they’re getting into.