、。
People consider it impolite to disagree with someone you don't know very well.
Literal
Too-much well know-each-other-not people [to-に] oppose-nominal [topic-は], polite-not thing [quotative-と] think [explanatory-のである].
A long written sentence with several teaching points. 知り合う ('get acquainted with each other,' literally 'know-meet') is a compound verb, and 知り合っていない is its ~ている negative — 'not in the state of having become acquainted.' 反対するのは nominalizes the clause with の and marks it as topic. The ~のである ending is a formal-written assertive, closer to 'it is the case that...' than to plain ~と思う — it frames the claim as an explained principle.