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Marriage and death are natural. For this reason, there is a natural, human aspect to weddings and funerals. They are offices (i.e., duties) of nature, dictated by the Natural Law.
Revealed religion is not necessary for men to know plainly (by their very nature) that marriage is for the coming into being, nurturing and education of children.
Here is how the Summa explains this truth, following the great philosopher Aristotle:
The union of male and female, which we call matrimony, is of natural law.
Further, the Philosopher [Aristotle] (Ethic. viii, 12) says that “man is an animal more inclined by nature to connubial than political society.” But “man is naturally a political and gregarious animal,” as the same author asserts (Polit. i, 2). Therefore, he is naturally inclined to connubial union, and thus the conjugal union or matrimony is natural.
I answer that, A thing is said to be natural ... to which nature inclines although it comes to pass through the intervention of the free-will; thus, acts of virtue and the virtues themselves are called natural; and in this way matrimony is natural, because natural reason inclines thereto in two ways. First, in relation to the principal end of matrimony, namely the good of the offspring. For nature intends not only the begetting of offspring, but also its education and development until it reach the perfect state of man as man, and that is the state of virtue. Hence, according to the Philosopher [Aristotle] (Ethic. viii, 11,12), we derive three things from our parents, namely "existence," "nourishment," and "education." Now a child cannot be brought up and instructed unless it have certain and definite parents, and this would not be the case unless there were a tie between the man and a definite woman and it is in this that matrimony consists. Secondly, in relation to the secondary end of matrimony, which is the mutual services which married persons render one another in household matters. For just as natural reason dictates that men should live together, since one is not self-sufficient in all things concerning life, for which reason man is described as being naturally inclined to political society, so too among those works that are necessary for human life some are becoming to men, others to women. Wherefore nature inculcates that society of man and woman which consists in matrimony. These two reasons are given by the Philosopher (Ethic. viii, 11,12).
Summa Supp., Q.41, a.1.
We also know that the Natural Law itself makes marriage for life.
Here is how the Summa explains this truth:
By the intention of nature marriage is directed to the rearing of the offspring, not merely for a time, but throughout its whole life. Hence it is of natural law that parents should lay up for their children, and that children should be their parents' heirs (2 Corinthians 12:14). Therefore, since the offspring is the common good of husband and wife, the dictate of the natural law requires the latter to live together for ever inseparably: and so, the indissolubility of marriage is of natural law.
Summa Suppl. Q.67, a.1, respondeo.
When someone embarks upon such an important new phase of his life, it is according to nature for us to celebrate with him and mark this important beginning with him.
Likewise, revealed religion is not necessary in order for men to know plainly (by their very nature) that we must reverence the body of a deceased loved one and that it is natural for us to “pay our last respects” as well as comfort the deceased’s family in their grieving.
Here is how St. Thomas Aquinas explains this truth, following St. Augustine:
As Augustine says (De Civ. Dei i, 13): “If a father's coat or ring, or anything else of that kind, is so much more cherished by his children, as love for one’s parents is greater, in no way are the bodies themselves to be despised, which are much more intimately and closely united to us than any garment; for they belong to man’s very nature.”
It is clear from this that he who has a certain affection for anyone, venerates whatever of his is left after his death, not only his body and the parts thereof, but even external things, such as his clothes, and such like.
Summa, IIIa, Q.25, a.6, respondeo.
Again, these are offices (duties) commanded by our nature itself.
The religious aspect of those occasions is layered on top of the office of nature. Therefore, a faithful and informed Catholic may attend a wedding or funeral religious service/mass of a compromise group or false religion, for natural, human, family or professional reasons.
But every faithful and informed Catholic must remember he is present for an office (i.e., duty) of nature and so participate in that aspect only. As these occasions are offices of nature, a person may stand when the bridal party (or the corpse) enters or leaves the church as a sign of this human, natural affection and regard.
But a faithful and informed Catholic would sit in back and attend the rest of the ceremony passively only, viz., no kneeling, standing, sign of the cross, shaking hands (i.e., the “kiss of peace”), responding, singing or participating in any way. Because weddings and funerals are offices of nature, a faithful and informed Catholic may send a gift or a card. He may attend the dinner (or party) and offer sympathy for the death (or congratulations for the wedding).
Other than weddings and funerals, there are no other occasions on which a faithful and informed Catholic would attend the religious services of any compromise group or false religion, e.g., a first communion, confirmation, ordination, “first mass”, etc. We should not send a gift or a card. We should not congratulate the person on his “big day” (first communion, ordination, or whatever). We should not attend the party, the dinner or celebration. Those occasions are not offices of nature and there is nothing to celebrate in them when the group is a compromise group or false religion.
When we decline to attend, we should explain that we cannot in conscience attend because there is nothing to celebrate. We should not make the cowardly excuse that we will be out of town (even if it were true) or otherwise are unable to attend because of sickness, scheduling problems, etc. Such excuses would make it appear that we would attend if circumstances had been different and we had been available. Those excuses falsely indicate we have no objection in principle to attending.
From the above explanation, it is also clear why it is wrong to attend the N-SSPX and Fake Resistance masses because they are religious services of compromise groups that involve no office of nature (which it would be permissible to attend as explained above, to the extent of fulfilling an office of nature).

The scandal involved

Besides offending God by attending a compromise religious service (viz., one involving no office of nature), your attendance further offends God because of the scandal you cause others by your presence and the support you thereby show for the group. This is true even if no one knows who you are, because humans are social creatures and they tend to accept what other people accept. Thus, your presence as one more anonymous attendee, scandalizes people because they see the bad group’s service is accepted by one additional person (you).
Lastly, joining with the N-SSPX (or other compromise group) for even one such religious event for whatever reason will tend to negate your efforts forever to help your family and friends go in the right direction. This is because ever-after, your family and friends will think that your future non-attendance is merely because those future occasions are not as important as the one you did attend and that you would be willing to attend future religious services of bad groups, when they are “important enough” to you.

Conclusion

We are soldiers of Christ! We must not compromise by attending any religious services of compromise groups or false religions, except for weddings and funerals (as discussed above, where an office of nature is involved).