One thing this might mean is that a person who is a male only child will marry a person who is a female only child. Again, it might mean that sons are the only persons who marry daughters and do not marry anyone else. Other interpretations are possible as well.
The process of rewriting a sentence so that one of its possible meanings be- comes clear is called disambiguating the sentence. One way of disambiguating a sentence is to rewrite it as a whole, spelling things out in detail. That is how we disambiguated the sentence “Only sons marry only daughters.” Another proce- dure is to continue the sentence in a way that supplies a context that forces one interpretation over others. Consider the sentence “Mary had a little lamb.” No- tice how the meaning changes completely under the following continuations:
1. Mary had a little lamb; it followed her to school.
2. Mary had a little lamb and then some broccoli.
Just in passing, it is not altogether obvious how we should describe the am- biguity in the sentence “Mary had a little lamb.” The most obvious sugges- tion is that the word “had” is ambiguous, meaning “owned” on the first reading and “ate” on the second reading. Notice, however, that this also forces alternative readings for the expression “a little lamb.” Presumably, it was a small, whole, live lamb that followed Mary to school, whereas it would have been a small amount of cooked lamb that she ate. So if we try to locate the ambiguity in particular words, we must say that not only the word “had” but also the word “lamb” are being used ambiguously. This is a reasonable approach, but another is available. In everyday speech, we often leave things out. Thus, instead of saying “Mary had a little portion of meat de- rived from a lamb to eat,” we simply say “Mary had a little lamb,” dropping out the italicized words on the assumption that they will be understood. In most contexts, such deletions cause no misunderstanding. But sometimes deletions are misunderstood, and this can produce ambiguity.