I'm single and I own a house. My entire domicile is my "cave".

Yes, but one day you might become not single, at which point it becomes her cave.

Some of the machine shop ones were cool, but that one with the stuff hanging in it like a lame TGIFs has to go. Man caves should have only a few themes:
1 - Sports
2 - Drinking and bar games, with sports thrown in.
3 - Workshops (wood, vehicles, firearms, and metal-working are allowed. Note that even though basketweaving may involve wood, this is not an acceptable man cave theme.)

The set-up should be somewhat dark or otherwise not female-friendly.

Suzy is not allowed, per Calvin.

While DensityDuck above is correct...those of us who are male and single, their entire abode is the ManCave.

But for me, the utter pinnacle of my nerdiness is the bedroom down in my--not my parents'--basement.

This is the room I call the "Geekitorium"...

Those who know me on this site know it to be true!

Ron: don't forget the prono!

Well, the pr0n goes without saying. I do believe I still have my laminated Snap On clock, complete with an early 80's beauty in some sort of lingerie. That will be hung in the man cave soon enough!

My man-cave holds crochet accessories and yarn of various colors. And a bench press.

What's not manly about that?

Well, it could be the crochet accessories. And the yarn. Plus the fact that you've more than one color of yarn.

Hmm. How about we replace "yarn" with "twine," "colors" with "tensile strengths," and "crochet accessories" with "recursive knot-tying tools."

And "bench press" with "decorative yet functional anaerobic exerciser with matching lace curtains."

Lemme see here. Still yarn. Still no power tools, machinery, sports, nekkid wimmens, or the substance of life (alcohol in itas myriad forms).

So, no.

Apologies. I don't make the rules, I just enforce them.