Making adult decisions "is" or "are" really not fun: Which is correct?

You are probably confused by adult decisions, which is plural. But that is irrelevant, as it is not the subject of the sentence. It would be if the sentence were

Adult decisions are really not fun.

Then, the plural are would be appropriate. However, in the sentence

Making adult decisions is really not fun.

the subject is "making adult decisions". The head of that phrase, making, is not plural. Thus, the phrase as a whole is not plural. Thus, is is correct (and are would be ungrammatical).

Making adult decisions is a singular noun phrase, as its head is the singular noun, making (a gerund). Thus, the singular is is the correct form to use here:

Making adult decisions is really not fun.

Alternative form:

The process of making adult decisions is really not fun.

In the second example, the subject of the sentence remains a singular noun phrase (the process of making adult decisions), as the singular the process is its head.

What everyone else said, and also: Change it to fishing, dancing, walking, cooking, or any other activity based on a verb that doesn't require an object, and it will be clearer:

Losing is really not fun.
-> Losing a dollar is really not fun.
-> Losing two dollars is really, really not fun.

The subject, "losing", is the same for all three. To convince you that "losing" is the subject in the last example and not "dollars", try it with a pronoun:
Calling me was the best thing you could have done.
"Me" can never be a subject, and you would never say, "Calling I was the best thing you could have done." Therefore, the subject is most definitely "calling".

Gerunds are somewhere in between a noun and a verb, as they are modified with adverbs and can take direct objects, but are always used where nouns would be:
He was praised for his efficiency.
->He was praised for efficiently handling the problem.

Compare that to "deverbal nouns," as they call them, which look exactly the same but require adjectives and no direct objects:
He was praised for his efficient handling of the problem.

Wild, ain't it?? Both perfectly correct and very common, both using the -ing form, but the only difference is deciding which noun/verb rules to apply to them.

Making adult decisions is a gerund phrase, which functions as a singular noun, and is the subject of the sentence. Gerund phrases are always singular, so the singular copula is appropriate.