The difference is mainly this: Orthodox marriages are invalidated by no “Catholic form” defect simply because the spouses are Orthodox, while SSPX marriages often fail (for Catholics) at the level of canonical delegation/faculty and thus at the Church’s required form, unless the Church supplies faculties for validity.

What makes a marriage valid (before we talk about Orthodox vs. SSPX)

At the root, Catholic doctrine holds that:

• Marriage is made by the consent of the parties: “The consent of the parties… makes marriage; no human power is able to supply this consent.” 1
• The most fundamental “baptism-related” invalidity is disparity of cult (between Catholic-baptized and non-baptized): “A marriage between two persons, one of whom has been baptized in the Catholic Church or received into it, and the other of whom is not baptized, is invalid.” 2

So in both situations (Orthodox and SSPX), if consent is truly present and the baptism conditions aren’t the specific ones that cause invalidity, the key remaining question becomes: what does Catholic law require when a Catholic is involved, and who may validly assist?

When Catholic canonical law “matters” for validity

Canon law is very explicit that if a Catholic is involved, the marriage is governed not only by divine law but also by canon law:

• “Even if only one party is Catholic, the marriage of Catholics is governed not only by divine law but also by canon law…” 3

In practice, this means the Church’s rules about the proper celebration/assistance required for Catholic marriages become decisive for whether a marriage by a Catholic is valid.

And there is a concrete example of this in the Church’s dealings with the SSPX: the Holy See provided a mechanism precisely to avoid uncertainty about whether marriages celebrated in SSPX circumstances are valid.

Why Orthodox marriages are valid (in the usual case)

Catholic teaching treats Orthodox Christians as non-Catholic Christians, not as “Catholics” bound to Catholic canonical form. From the perspective of the sources we have here, the “validity engine” is:

• Consent makes marriage (so the sacramental/natural validity is primarily about what the parties do/agree to). 1
• Catholic canonical discipline about “marriage of Catholics” applies specifically when the marriage is that of Catholics (i.e., when a Catholic is involved). 3

So, if two Orthodox persons (both baptized) marry with real matrimonial consent, their marriage is not invalidated by a failure of Catholic canonical delegation/faculties, because—unlike SSPX weddings involving Catholics—it is not the Church’s “Catholic marriage form for Catholics” that is being tested.

Why SSPX marriages are not automatically valid (and why many are)

The SSPX situation is different because the question often is not merely “did the parties consent?” but whether the marriage was celebrated in a way the Church requires for Catholics—i.e., with the necessary delegation/faculty/assistance.

SSPX ministers’ canonical irregularity

The Church has taught that, until the doctrinal questions are resolved, the Society lacks canonical status and its ministers do not exercise legitimate ministry:

• “As long as the Society does not have a canonical status in the Church, its ministers do not exercise legitimate ministries in the Church.” 4
• And: “Until the doctrinal questions are clarified, the Society has no canonical status in the Church, and its ministers… do not legitimately exercise any ministry in the Church.” 4

That matters because (for Catholic spouses) the Church requires marriages to be celebrated according to its norms; if the ministerial assistance is not what Catholic law requires, validity can be at risk.

The Church provided faculties precisely to secure validity for SSPX-linked marriages

In 2017, the Pontifical Commission Ecclesia Dei/CDF letter states that Rome authorized ordinaries to grant faculties so that marriages of faithful attached to the SSPX can be celebrated with conditions intended to ensure validity (not merely liceity):

• The Holy Father “decided to authorize Local Ordinaries the possibility to grant faculties for the celebration of marriages” of those who follow SSPX pastoral activity. 5
• The delegation is described in terms that allow the SSPX-associated priest (or a regular diocesan priest) to receive the parties’ consent in the marriage rite, following the intended liturgical practice—explicitly “such that… any uneasiness of conscience… and any uncertainty regarding the validity of the sacrament of marriage may be alleviated.” 5

So the Church’s own logic is: without those faculties/delegations in the right way, validity can be uncertain; with them, validity is meant to be secured. 5

Delegation/faculty is treated as legally required, and absence is serious

A Rota decision (2009) addresses the SSPX context in terms of delegation to assist at marriages and emphasizes that delegation must be real and can’t just be presumed:

• It notes the DDF’s view that, absent special norms, SSPX priests “assist validly… provided that the norms foreseen by common law (cann. 1108-1111) are respected.” 6
• It also explains delegation requirements and legal character: “Delegation… is a fact, and facts are not presumed but must be proven.” 6
• And: “If in the marriage act there is no mention of delegation, then defect of delegation is rightly presumed unless the contrary is proven … especially when the marriage act simply lacks that information.” 6

This is why people often say “SSPX marriages are not valid”: in many real cases, the canonical documentation/delegation required for a Catholic marriage to be celebrated according to Church law simply was not present in the act—making the marriage vulnerable to a validity challenge. 6

So what’s the clean bottom line?

• Orthodox marriages: validity primarily depends on real matrimonial consent (and baptism conditions). The Catholic “extra” canonical-form questions that bind Catholics don’t automatically apply in the same way to Orthodox-only marriages. 1 3
• SSPX marriages: validity for Catholics often hinges on whether the SSPX-linked celebration had the necessary faculties/delegation and whether the act satisfies the Church’s canonical requirements. The Holy See has repeatedly had to supply faculties to avoid uncertainty about validity. 5 4 6

The statement “SSPX marriages are not valid” is therefore not universally true in every case, but it reflects a real canonical risk: without the Church’s faculties/delegation in the proper way, a Catholic marriage celebrated in an SSPX context can lack what Catholic law requires for validity. 5 6