There are so many implications when it comes to this. Besides what people have shared, you have to think about some cultural/governmental things in Argentina.
There is a lot of red tape and what would be "easy" here won't be easy over there. The having a child out of wedlock could get complicated in Argentina and the USA if you are trying to naturalize them by having a US-citizen father.
I know getting married could delay it by a month or two, but it might be a smoother ride. If you are concerned about the responsibility of the I-864, think about the potential of doing a pre-nup and just talk to your S.O. to see if they would agree to become naturalized as soon as they are able, etc. Again, none of this is a guarantee. Props to you for being realistic and at least talking about it. I know some people might jump on the "it is a red flag" if you are thinking about the failure already. But, I also think that if your relationship is healthy, it is good to just be open and talk about all the possibilities so you could be on the same page no matter the outcome.
We have a Whatsapp group of Argentina filers. If you are interested in joining, feel free to inbox me.