As "coming out of the closet" has become ubiquitous in recent years, what would the plural be? Would it follow the rule of "goings-on" or be a hyphenated "coming-outs" or something else entirely?
Often coming out is a verb form rather than a noun
We bit the bullet and are coming out to our parents today!
No plural necessary.
Often it is an adjectival form
We wanted a coming out party to share our happiness with the world.
No plural necessary.
Maybe when the gerund stands by itself, it should be considered an uncountable noun, like beauty or freedom
After the Supreme Court decision, hundreds of couples celebrated their coming out in style.
Hot Licks already posted the answer as a comment, but comments can be expunged, so here it is as an answer:
No idiomatic answer has been established You either wing it or tiptoe around it.
As RegDwigнt♦ posted in yet a different comment, there are many examples in English where the entire hyphenated phrase is treated as a single unit for pluralization:
Yes, it's passers-by and not passer-bys. But it's also break-ups not breaks-up, blow-outs not blows-out, strike-outs not strikes-out, merry-go-rounds not merry-gos-round, etc. etc. (people in chat list dozens more). And yeah, coming-outs and not comings-out.
Google n-Grams shows that comings-out has been in relatively widespread use for much longer than coming-outs:
Note that the graph shows a dramatic increase in the use of coming-outs in the last decade of data. If you click through to the results, you'll see that these are, indeed, true positives, where writers are using the phrase to refer to multiple instances of a closeted person revealing their secret (sexuality is the typical secret). Given that in much of the English-speaking world, gay people have had more coming(s)-out(s) recently, it's normal to see a spike in the usage of the phrase, but note that the overall distribution of the usage is almost evenly split between the two camps.
If there is a rule which determines where the pluralization is placed, it doesn't seem to be based on the "importance" of the component word, but rather on the component words' functions and whether or not the whole hyphenated phrase is taken as a single entity or not. I think there is more to this but haven't the time to look into it further.
TL,DR: Use comings-out if you want to be fussy. Use coming-outs if you want to be modern or if you feel strongly that the entire thing is one single unit word spelled with the hyphen letter.
The correct plural is comings-out. Cf. passers-by, runners-up.
Grammarphobia.com gives the following rule: “• If the word is split into parts, with or without hyphens, put the plural ending on the root or most important part:
mothers-in-law - courts-martial - ladies-in-waiting - hangers-on - crêpes suzette - rear admirals - men-of-war - flagships - tank battalions - Johnnies-come-lately (or alternatively Johnny-come-latelies)
“• Watch out for general when it’s part of a compound word. In a military title, general is usually the important part, so it gets the s. In a civilian title, general isn’t the root, so it doesn’t get the s:
Two attorneys general went dancing with two major generals. Those consuls general are retired brigadier generals.”