Why is this joke about elephants and quarts funny?

I feel like I'm walking into some kind of trolling here, but I believe the joke is based around the definition of "comes" which is synomymous with "ejaculates".

In other words, the joke is implying that volume of the elephant's ejaculate is rather large: a "quart" of course being another word for a quarter of a gallon, aka two pints.

The first-glance reading of "comes in quarts" would be "is available in quarts" (like milk), and the reversal of the understood meaning of "comes" is where the humour arises (if it does arise).

It seems it's some kind of "penis joke", as explained above, and it seems it doesn't have much sense anyways, as outlined by @march-ho:

According to this paper (SFW), elephants ejaculate 5-75 ml depending on the type of ejaculate, or less than 10% of a quart.

Based on other opinions, those elephant jokes had a lot of symbolism.

Based on Wikipedia:

Elephant jokes are seen by many commentators as symbolic of the culture of the United States and the United Kingdom in the 1960s ... Abrahams and Dundes, in their paper On elephantasy and elephanticide, consider elephant jokes to be convenient disguises for racism, and symbolised the nervousness of white people about the civil rights movement. Whilst blatantly racialist jokes became less acceptable, elephant jokes were a useful proxy. Abrahams and Dundes take the joke

Q: What is big and grey and comes in quarts?
A: An elephant.

and state that the "big and grey and comes in quarts" is in fact a reference "to the supposed mammoth nature of black sexuality." Similarly, the joke about an elephant in the bathtub is argued to be a reference to the increased intrusion of black people into "the most intimate areas of white life."