About yoking with commas
This is wrong but why?
No, the answer is not simply that it is another example of a pedant inventing reasons for pedantry that isn't based in anything real, although that's true. It's not even that the Oxford comma is an archaism that most writers in English no longer use (I'll have more to say about that later).
It's this. With one exemption, you may not yoke two items (or if there are more than two items yoked, the last two) in English with a comma. You must use "and".
Here are some examples of incorrect yoking:
"I like to eat fish, meat."
This is just not grammatical in English and you must write "I like to eat fish and meat.". It's more clearly seen in:
"I like to play football, rugby and watch television."
This is common but "and" is needed between "football" and "rugby" because otherwise you are saying that you like to "play watch television".
"I hit him, he fell over."
This is a runon sentence because you must yoke coordinate clauses with "and".
"I fucked her long, hard."
I think you're getting the idea by now.
In the example given, for the sentence without the Oxford comma to be wrong, we would have to be yoking "eggs" and "toast and orange juice". This is not allowed because this is a parallel to "I like to eat fish, meat".
To make the sentence grammatical you must write "I had eggs and toast and orange juice." In written English this can only mean that you had two items. Why? Because not only must you use "and" to yoke the last item in a list, you may only use it to do that. Unless you are five years old.
As for the Oxford comma, it is simply an archaism from a time when English used a comma before every conjunction. On the whole, Americans still do. They probably should use serial commas because they use a comma before "and" in a sentence such as "I kicked him, and he didn't like it." or "I went to the shops, but I didn't see him." where an English writer would not (should not, in case any English writers read this and do use it). There are occasions for an English writer to use a comma before a conjunction but when you're conjoining simple clauses isn't one of them.
But didn't I say there was an exception? Yes. You do not always yoke adverbs with "and". In lyrical writing of the kind that mostly bad writers indulge in, you can see things such as "it fell to the ground slowly, softly, relentlessly". Although a good writer eschews this sort of thing, it's not strictly incorrect. Triads are a common device in writing, which are useful but should be deployed with care. "He's mad, bad, dangerous to know" just doesn't read well even if you want to argue that it's not strictly incorrect. Indeed, this is an example I would certainly correct, although I wouldn't go to the mat for the correction.
How about "Men, and women too, dislike eating snails."? Here we are using a parenthesis. When we do this, we must close the parenthesis with a second comma. "Men, and women too dislike eating snails." is not grammatical. "Men and women too dislike eating snails." is grammatical but reads a little awkwardly. BTW, do not parenthesise or use a comma before "too". People do, I know, but they are wrong to.