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 singular. Thus, the phrase as a whole is singular. Thus, is is correct (and are would be ungrammatical).

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).