Step 1: Notice the word compels.
The question uses the same word as the passage. Find the sentence that holds it.
Step 2: Read the subject of that sentence.
The thing that compels is the securing of several wives by one or two leading persons.
Step 3: Reason on the numbers.
In a band of under twenty people, if two men take three or four wives each, few women are left. The remaining men cannot marry.
Step 4: Match and conclude.
Only option 2 gives this cause. The rest give choice, poverty or parents, none of which appear in the text.
Final Answer:
Some men stay celibate because a few leaders take several wives.
\[\boxed{\text{Option 2}}\]