A “dress watch” must be on leather (or some sort of vegan equivalent).

This is obviously true and anyone that says false is either a politician or a carny. 

Nonsense. Ribbon straps are fine too. 

Paging @Scaramanga  who did stuff like this all the time

(which admittedly is beyond my comfort level)

The "rules" say yes, but...

Asking for a friend: would something like this look foolish?

Eh, there are other straps that are dressy as well. For instance my dress watch is on a metal strap, which doesn't look any less dressy than leather to me.

I don’t think so. personal styles may vary, but I could see that working with a summer suit.

“it is a custom
More honored in the breach than the observance” - Hamlet

I am the third generation of professional curmudgeons who wear watches on bracelets with suits. If I go through old photos of people with style and class, this also wasn’t unusual for them. I’m not sure when this rule ever came about, but it seems like a recent contrivance to me.

So, yes.

I voted "False" because many mid-century watches were meant to be worn with metal bands (I am thinking of Hamilton with Kreisler bands). @Scaramanga can pull off NATOs because his straps are more expensive than many of our watches.

I prefer leather, specifically something thin and textured, like lizard or something more exotic. But, it is just a preference. The ISSSRWC has not weighed in.

Here’s an article for all you uncouth beatniks:

https://www.gentlemansgazette.com/black-tie-watch/

Articles like this fail to differentiate between suits and tuxedos. (How often do any of us wear a tux, really?).

Even in a tuxedo context, if I see someone at the Oscars with a day date on, I’m not shaking my head in disbelief. Same if I see a groom with a nice watch on a bracelet for his wedding day.
 

And do these rules apply equally across genders? 🤔

@Bobofet I know that as an intergalactic bounty Hunter, an all beskar steel watch seems appropriate to you. Hell, you mandalirians won’t even take your helmet off in public!  For the rest of us that aren’t hiding out on Tatooine, we long for a more civilized era!

you mandalirians won’t even take your helmet off in public!
 

That only applies to a small group of fanatics…

Yeah but you all wear steel divers as dress watches. Just as bad. 

Blancpain has built a business model around it.

The author of that article should be forced to wear an orange G-Shock to his daughter's wedding. It is an abomination.