Welcome to the Forum, English Maiden!
First, let’s consider the word “destiny,” which could either be a countable noun or noncountable noun depending on its usage. It’s countable when it denotes what will happen to somebody in the future, particularly outcomes that he or she can’t change or avoid. On the other hand, it’s noncountable when it refers to the unknowable entity that’s believed to have the power to control events.
Now, in Sentences A1 and A2, “destiny” is obviously being used in the countable sense since it refers to the individual destinies of the people who comprise the plural pronoun “we.” Sentence A1, which pluralizes the count noun “destiny” into “destinies,” is therefore the correct sentence construction: “We are the masters of our own destinies.” Sentence A2, “We are the masters of our own destiny,” is grammatically and semantically wrong.
Let’s tackle the term “sex life” next. Obviously, the noun “life” is being used in the context of “human activities,” so it’s definitely a countable noun. This is in contrast to the use of “life” as a noncountable noun in the sense of the animating and shaping force in living things. The term “sex life,” then, is a countable noun that should be pluralized to “sex lives” when it refers to the individual sexual activities of the people who comprise the plural pronoun “we.”
Based on this grammatical analysis, Sentence B2 is the preferable sentence construction: “People with diabetes can still have normal sex lives.” I say preferable because Sentence B1 is also grammatically and semantically correct: “People with diabetes can still have a normal sex life.” Unlike the noun “destiny,” which is a countable noun that pertains solely to a particular individual, “sex life” can also be construed as a plural collective noun that pertains to people in general.
What this is telling us, English Maiden, is that whether a countable or noncountable noun will be plural or singular also depends on the nature and attributes of the noun itself and not just on its grammatical usage in the sentence.