Why Can't 'a Wife' Walk Down the Street?

Ever since the sentence 'a girlfriend was walking down the street' appeared to me unbidden in a hazy late-night half-sleep, I've been trying to understand what makes certain relationship words sound so weird as the subject of a sentence. Consider:

• 1a."I saw a boyfriend enter the store."

• 1b. "A wife selected zucchini from the produce section."

• 1c. "The dog chased a husband down the street."

My immediate thought is that these relationship words, girlfriend/boyfriend/wife/husband, need a pair to make sense. The sentences are immediately made more acceptable by adding on another member:

• 2a. "I saw two boyfriends enter the store."

• 2b. "A wife selected zucchini from the produce section and handed it to her husband."

• 2c. "The dog chased a husband and wife down the street."

Compare also the words 'mother' and 'father'. Both of these need another member to make sense ('child'), but it is actually quite acceptable to use them by themselves, in the indefinite.

• 3a."I saw a lonely dad at the playground."

• 3b. "A mother was excitedly waiting in line for coffee."

Hmm, what if we compare with another relationship word, 'friend'?

• 4a. "A friend entered the store."

• 4b. "I saw a friend select zucchini from the produce section."

The effect is interesting - I read these sentences differently from the ones above. The implication is very strong that this is my friend, not somebody who is a friend to someone else and not me. Meanwhile, sentence sets 1 and 2 don't have the implication that the boyfriend, wife, or husband have any relation to me at all. While the feeling is uncanny, you do get the intended meaning, which is "somebody who is male who is in a committed relationship entered the store." If you try to force the same distant reading on the 'friend' sentences, you get the same quirky feeling.

I suspect part of it is custom - we almost never have need to refer to someone's paired-off status without mentioning the pair. On the other hand, people talk about the behavior of mothers and fathers separate from their children often. And to refer to a stranger as a friend of some unmentioned other person, but not you, is something we almost never need to do. Don't we assume, rightly or wrongly, that everyone is a friend of someone else?

This obviousness comes into play with 'daughter' and 'son':

• 5a. "A daughter came into my store."

• 5b. "A son selected zucchini from the produce seciton."

Everyone is someone's child, so this construction that foregrounds someone's status as a son or daughter is simply unneeded.