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).