What does "You would / I did / But really" mean?

Question

In "The Boys of Bummer", the 18th episode of The Simpsons' 18th season, Homer trades his matress for the Lovejoys' because the former is supposed to be afrodisiac. Later on he wants it back, so he and Marge sneak into the Lovejoys' house to take it when the Lovejoys arrive home. They have the following conversation (16:55):

Marge: Our marriage needs this bed.
Mrs. Lovejoy: Our marriage needs it more.
Mr. Lovejoy: Well, I am reminded of the story of wise King Solomon.
Homer: Pff, you would be.

I am confused about the last line. "You would" seems to be some kind of set phrase, but I cannot seem to find it on any dictionary I've checked. The only relevant entry is on Urban Dictionary:

You WOULD. An extremely anoying phrase meaning "that is very typical of you".

That makes it a little bit clearer. I assume Homer is saying he isn't surprised Mr. Lovejoy, being a reverend, brings up a story from the Bible. Is his remark meant to express his irritation, as in "You couldn't leave the Bible aside just for once, could you?"?

Urban Dictionary goes on to say:

Commonly seen following the equally agitating expression "but really". It is a well-known fact that you cannot win a logical battle with the "you WOULD" person. The only known comeback to "you WOULD" is "i DID", but does not sufficiently bring down the person. You must emphasize "WOULD" or it loses meaning and sense completely.

Person- "omg i totally forgot we had a huge test today!"
Idiot- "you WOULD forget we had a huge test today."
Person- "i DID forget."
Idiot- "but really"

Now I'm even more confused. I understand neither "I did" nor "but really". If Idiot (let's call them B) is saying Person (A) usually forgets about tests, why would A answer "I did forget"? Isn't that what B is saying? How could "I did" be a good comeback? And what is "but really" supposed to mean? Does B really think A forgot about the test? What is going on?

Answer 1

“That is typical of you” is exactly what “You would!” means in this context. It is listed in some dictionaries, though it can be a little hard to find among the many other meanings of would; for example Cambridge lists it as

would modal verb (FREQUENCY) (…) disapproving used to ​suggest that what ​happens is ​expected because it is ​typical, ​especially of a person's ​behaviour:

"Madeleine called to say she's too ​busy to come." "She would - she always has an ​excuse."

This is closely related to the meaning explained just above: would can express something habitual. For example

He would always ​turn and ​wave at the end of the ​street.

means the same thing as “He always turned and waved …” but the use of would stresses the fact that it's a habit.

As for what you read in Urban Dictionary — don't mistake UD for a useful reference. A lot of what it contains is just what one person decided to add and doesn't mean that anybody else would understand. I, for one (admittedly not a native English speaker), have no idea what why “you would” is supposed to be annoying or this “I did forget”/“but really” business is supposed to mean (maybe it implies that “you would” was used sarcastically?).