Student or Learner
"I want to marry a Frenchman. "
This sentence doesn't mean "I want to marry any Frenchman.", doesn't it? The speaker has one particular man in her mind, right?
Without further context, I would assume that she has an affinity for French men and would like to marry one, but this doesn't mean that any French man will do, nor does it mean that she knows him already or has a particular man in mind. Fiona's interpretations are equally valid, so it's hard to say. If she is in desperate need of a visa, then any French man will do.
Here is a serious reply.
Consider the following
a) I want marry a man
b) I want to marry any man
c) I want to marry some man.
In a) 'a man' refers to an unspecified member of the category 'man/men'.
Which man may be unspecified either because you haven't yet decided which one to marry or because, even if you have, you are not giving this information to me in your message.
a) may seem to be an unatural utterance because if you are a woman there may seem to be little point in giving me this information in the first place.But if you are a man then the message is highly significant.
Assuming you are a woman, 'a Frenchman' seems to be a more natural than 'a man' since you are selecting a member of a particular subcategory of men rather than a gender.
b) 'any' refers to the entire category of men. 'Any man' means it doesn't matter which one. They are all OK.
c) 'Some' specifies a part of the category and excludes the possibility of everyone.
c) would be regarded as ungrammatical by many teachers but it is common enough in colloquial speech. e.g 'Some guy told me that ...'
In short 'I want to marry a Frenchman' means 'I want to marry a man who is French', which is leaves the question of whether or not you have already reached a decision unreported, unlike,'The man I want to marry is French'.
I assure you that my reply was also serious! It even results in the same conclusion as yours: "In short 'I want to marry a Frenchman' means 'I want to marry a man who is French', which is leaves the question of whether or not you have already reached a decision unreported".