trivial plural

As a student we formerly had to choose between singular and plural:

-We did not have to remember the faces of all males we came across in our __ to know what 'male' meant.

The model answer was lives, not life. If you wrote 'life', you got no marks. My opinion: if you are saying a group of people each having the thing you are talking about, you usually use the plural. But whether we need the plural sometimes depends on the word itself, so we say their minds, not mind; make their way to..., not ways. If a student wrote 'life', do you think he would be correct and should get the mark? (For example, at the beginning of this thread, I said 'as a student we...', I think sometimes the seemingly illogical plural is acceptable.)