Not necessarily. A culture that include the concept of a "raiser" - an octopus with the job of raising the babies, and passing the culture on to them, without mating at all - can avoid that issue. The "raiser" would also improve his average genetic fitness if he is a sibling of one of the parents, since the children would then all have approximately one-quarter of his genes.

(it's unclear why this evolved)

If it's not enough to kill off the species, evolution generally won't drop the feature.

How about "If you know nothing and are willing to learn, you're closer to the truth than someone who's attached to falsehoods"? Even then, I suppose you'd need to throw in something about the speed of learning.

I think it may depend a lot on how well the action fits into your schema for reasonable behavior.

I have mild OCD. Its manifestations are usually unnoticeable to other people, and generally don't interfere with the ordinary function of my life, but occasionally lead to my engaging in behaviors that no ordinary person would consider worthwhile. The single most extreme manifestation, which still stands out in my memory, was a time when I was playing a video game, and saved my game file, then, doubting my own memory that I had saved it, did it again... and again... and again... until I had saved at least seven times, each time convinced that I couldn't yet be sure I had saved it "enough."

Afterwards, I was horrified at my own actions, because what I had just done was too obviously crazy to just handwave away.

http://blog.yunwilliamyu.net/2011/08/14/mindhack-mental-math-pseudo-random-number-generators/