The una cuм argument is an error, but it is not against the faith. It is correct as far as pastoral theology is concerned.

That makes no sense.  I've been told personally many times, and have heard it from the mouths of well-known sede clerics on videos, that those who go to 'una cuм' masses commit a mortal sin.  If this is an erroneous stance, (and it is, because it is not from the Church, but is invented), then such a error is also mortally sinful, since its accusations, it's scandal and its topic are of the same moral gravity.  

What if someone called something a mortal sin for a certain reason, but it was wrong....but nevertheless was a mortal sin for another reason?

This is the case with the "una cuм" threat. It is wrong in considering it a mortal sin for an intrinsic reason, but is still correct for considering it a mortal sin for an extrinsic reason. Here is the quote that proves it:

"Since heresy, and any kind of infidelity, is a mortal sin, they also sin mortally who expose themselves to its danger, whether by their association, or by listening to preaching, or by their reading." - St. Alphonsus Liguori

Why don't you answer in plain english?  The above answer rivals a Wessex riddle.  If one attends an 'una cuм' mass, is it a mortal sin?  Y/N?  If so, why?  Show me from canon law, from a papal pronouncement or even from one of your much-beloved randomized saint quotes which usually are taken out of context.

If one attends an 'una cuм' mass, is it a mortal sin?  Y/N?  If so, why?  Show me from canon law, from a papal pronouncement, etc.

"Work of Human Hands"

I watched the multi-part videos that Fr. Cekada put on Youtube, but have yet to read this book. Would you say it is a must read?

.edit.

Bumphrey,
Why can't you answer my question?  I'm not trying to trick you; it's very simple.

If one attends an 'una cuм' mass, is it a mortal sin?  Y/N?  If so, why?  Show me from canon law, from a papal pronouncement, etc.

You seem to imply that those priests who say an 'una cuм' mass are not intrinsically wrong but it is wrong based on circuмstances.  What circuмstances are those?  Just the fact that they mention the pope's name, therefore they are praying 'in union' with him (and his errors)?  Look, i know the argument.  What I want to know is - show me where the Church has ever taught this, based on the una cuм prayer ALONE.

Actually, I have answered you, but you didn't understand it.

I said it was not intrinsically evil. Let me clarify.

Let me give you an analogy in normal times about the meaning of that. It means that something is likely an occasion of sin to the average person, so, in general, it is forbidden as a mortal sin, because to knowingly put oneself in a circuмstance of near occasion of sin, is itself a mortal sin even if the person ends up not sinning mortally....because he willfully took a chance of sinning grievously.

But there could be individuals to whom it is not a near occasion of sin. Catholics are not good judges of whether it is or not, so it should be a confessor, or higher Church authority to decide. For instance, a Catholic, by canon law, would be excommunicated for even having an heretical book on his bookshelf. However, permission or dispensation could be given in a case of necessity to a learned Catholic to be able to read such a book.

Basically that is what a "dispensation" is. It is a circuмstance that is dangerous to the average person, and so forbidden to all, UNLESS a special permission is given based on evaluating the person and circuмstance.

So, to apply this again...it is a near occasion of sin for the average person to attend an R&R Mass because of the magnetic danger of being attracted to whatever the priest is associated with.