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In your experience, are priests during the pre-Cana preparation, explicitly clear …
Yes. Of course, we are.
The reason I ask this is …
Fair enough. That might need some explaining.
In the U.S. we have standard pre-marriage forms. These will vary by diocese, but they are all essentially the same. A question might be #12 for one diocese and #27 for another. The exact details vary, but the important questions are all asked. If I understand correctly, the same happens in Canada. I don’t know how these questions are posed to the couple in other cultures, especially ones with lower literacy rates. Maybe in some places, it’s all verbal, for all I know.
What I do know, and what I can say with absolute certainty is that it is always-and-everywhere the pastor’s responsibility to conduct a pre-marriage investigation into the status of both parties. He must perform his due diligence. Anything less that that is plain negligence. It is his responsibility to know, for example, if the man has another wife in some other city. He must ask. He must follow whatever procedures and practices are appropriate for the culture and the specific issue to know that both parties are eligible to enter into marriage before he officiates at the ceremony. Now, there are several different ways that can make one or both of the parties ineligible to marry. He must make every reasonable effort to know whether or not any impediments to marry might apply.
In the U.S. we do that by completing forms. Some questions (what is your current address?) don’t require explanation. Some do require explanation. It’s the pastor’s responsibility (even if he delegates the actual preparation to someone else) to make sure that the couple understands the questions. In most circumstances (where some explanation is needed), simply explaining the question suffices. If someone asks “what’s this word ‘conjugicide’ mean?” a simple “did you ever murder your wife?” is enough. Future-groom checks “no” and moves on to the next question.
Moving more directly to the topic at hand: when this question is asked, something like “are you capable of engaging in the marital act?” someone might not catch the meaning at first. A simple and polite explanation then follows.
Remember that the priest doesn’t just hand the couple some forms and tell them to complete them then return them to the office the next day. The pastor himself, or someone who is trained and qualified, walks the couple through the process. That’s essential to know. The couple is not at all “left to their own understanding” as you asked in your question. If that were the case, we would either be getting a lot of wrong answers, or we would have to completely change the forms to include rather lengthy and cumbersome explanations.
So don’t worry about that. We don’t just tell the couple to fill-out some forms. The forms are just one step in the process and a qualified person, whose very job is to provide explanation, is always part of that process.