I disagree. If you pick up a bag at the airport that you believe to be your own, it’s not theft if it turns out to be someone else’s.
Anyone may of course disagree with a comment of mine, but it is not necessary to cast aspersions while doing so. It is not that I don’t understand your viewpoint, but rather that I am not quite so sure of the concept of a venial sin of adultery. It is the jist of the OP, and so is it then not fair to question the proposition?
Beyond that, you have said several times that perhaps you have not understood my comments. I have merely agreed. That you have used “straw man” arguments is there for all to see. Those comments of mine have nothing whatever to do with any other poster.
Thomas I have no idea why you believe I am casting aspersions on your motives

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I have bent over backwards to politely demonstrate that your difficulties with the very concept of a venial adultery is due to a mistaken definition of what Catholics mean by the word adultery.
I now fully understand your position and why your questions are therefore inherently unanswerable. It is because your starting point (a correct definition of the word adultery) is mistaken. As Aquinas says, a small error at the start leads to large errors at the end.
If you believe that all swans are white then when explorers found black swans in Australia
you might respond as those British philosophes did, “that’s not possible, they must have seen large ducks.” A logical but mistaken conclusion due to an erroneous starting assumption.
So there is little more I can do to assist you in your difficulties re venially adulterous irregulars
if you cannot accept what I and others are observing re your faulty definition of adultery.
Re “theft” you are of course correct, your appropriation of the bag was not theft. “Theft” happens to be a word like “murder”, it is by definition pejorative of the agent. It essentially means intentional illicit appropriation and is hence understood to always be morally wrong.
“Appropriation” or “take what is not yours” on the other hand is morally neutral and objective and better describes the “grave matter” specified by the 7th Commandment. It is thus neutral like “killing”.
So regardless of your understanding or level of consent (perhaps a terrorist held a gun to your head) you have indeed appropriated the bag. Hence this physical evil could be neutral or venial or mortal moral evil…we cannot tell without being inside your head. All we can see is the objective physical evil you did which remains the same,physically evil, in all three moral scenarios. We suggest “irregulars” observed in the physical evil of adultery are in exactly analogous situation. Obviously the physical evil is greater. Someone else’s wife has been appropriated.
Just as the Magisterium has in more recent times insisted on translating the 5th as about killing rather than murder perhaps it is time we did the same for the 7th, translate as Thou shall not take what is not thine.
Perhaps it’s time the 6th was re translated “Thou shall not be irregularly married.”
Or, as adultery is actually a form of unjust taking, perhaps “Thou shall not take another’s wife.”
Then the “grave matter specified by the Commandments” would not be confused with words understood by some to mean mortal sin as you have done here.
Obviously in times past when priests referred to the Commandments as listing “mortal sins” they clearly meant “objective” component of mortal sins. Most of us get this because if we don’t then the CCC would be making erroneous statements. Like suggesting the Commandments may be committed venially.