You’re misconstruing the argument, then. And, since you clearly have some skin in this game, it’s not surprising that you seem to be hearing parts of the discussion and missing others. (At least, that’s how it looks to me as I read through this thread.)
The argument being made here isn’t that “marriage boils down to sex alone.” Let’s look at the hypothetical situation: man and woman marry; they divorce; without receiving a decree of nullity, one of the spouses remarry.
Now – are we saying that the status of the marriage has anything to do with “sex alone”? Of course not!
But – and this seems to be the point that you’ve glossed over – what’s the sin that is being committed by the remarried spouse? Is it a sin of ‘marriage’? No … since (presuming the validity of the first marriage), this spouse isn’t seen by the Church as being validly married. Therefore, what is the nature of the sin being committed by the spouse? It’s the sin of adultery: they are sinning by having sex outside of a valid marriage.
(Does this imply that there was no sin in the remarried spouse’s abandonment of his (first) marriage? Of course not. However, isn’t it possible that he has been to confession and been forgiven of the sins that he committed in that marriage? Of course it is.)
Therefore, the sin that we are considering is the sin that – to all appearances – is being committed on an on-going basis: illicit sexual activity. This doesn’t imply ‘legalism’ or “boiling down marriage to sex,” even to those who wish to describe the situation in an excessively emotional way…
Fine. But what if you’d been to confession and been absolved of the sins you committed in leaving your husband? Wouldn’t you be considered to have been forgiven?
Which, after all, is why the notion of scandal is so critical to this discussion. If there’s the possibility that there is public knowledge of ‘play acting’, as you put it, then the possibility of scandal exists, and it would seem that the ‘pastoral solution’ would not be feasible in this case.
You read dans0622’s post, didn’t you? That’s precisely the issue being raised there – the problems between the public nature of the (subsequent) relationship and the private nature of the way that the relationship is actually being lived out…