天经地义-------a matter of course

例句：
A: Don't you think it a matter of course to respect our elders?
难道你不认为尊敬长辈是天经地义的事吗？
B: Sure. However，a lot of young gays don't think so.
当然是天经地义的事。然而，许多年轻人并不 这么认为。

同义词：
a universal truth
It does without saying...