particle の - Is ending question sentences with の really feminine?

In What differences should I look out for between male vs female speech?, a lot of answers explicitly mention that ending question sentences with の is feminine.

However, this makes very little sense to me, as my understanding is that の is pretty much the go-to particle for forming questions with non-polite speech.

In general, か sounds pretty marginal to me with non-polite speech:

(1) ｘ何をしてるか？
(2) ｘ学校に行くか？

Sometimes, のか works:

(3) ｘ何をしてるのか？
(4) ○学校に行くのか？

(I think のか only fails when you have a 'question word' like 何, どこ, だれ, etc.; it seems to make it sound rhetorical, e.g., 「彼女は何をしてるのか、僕は分からない。」)

And of course の always works:

(5) ○何をしてるの？

(6) ○学校に行くの？

And as far as I can tell, simply using intonation (or a question mark to represent it) works all the time:

(7) ○何をしてる？
(8) ○学校に行く？

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First off, my judgements here might be wrong since they are just my own.

But, if they are correct, my question is: if ending your sentence with の really is feminine, what is the alternative to (5) in usual male non-polite speech? Is (7) the only option?

Answer

I think that in some cases, ending a question in の is fine for male speakers. For example, I hear

え～、そうなの？

quite often from male speakers. I think, in general, we have that (all male speech)

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  rhetorical questions are allowed to end in の, e.g. even if it is clear what the other person is doing, you may ask

  
  

  
  
  

  何をしてるの？ or 何してんの？
  What (the heck) are you doing!?

  
  

  
  


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  questions as honest inquiries don't end in の, e.g.

  
  

  
  

  今何をしてる？
  What are you doing right now?

  
  

  
  
  


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  か is usually used for rhetorical questions addressed to oneself, e.g.

  
  

  
  

  行くか。
  I guess I'll get going.

  
  

  
  


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  のか is more for rhetorical questions addressed to other people, e.g.

  
  
  

  
  

  学校に行くのか。
  I see, you're leaving now, are you?

  
  

  
  

Decidedly feminine is when you end a statement in の, e.g.

そうなのよ。
知らなかったの。

with the male equivalent being either nothing (e.g. そう, 知らなかった) or んだ(よ) (e.g. そうなんだよ, 知らなかったんだよ).

What male speakers would express as a clear statement (nominalized sentence with contracted nominalizer and declarative だ) is softened in female speech by omitting the "strong"-sounding parts, viz. の is left uncontracted and だ is omitted.