In the paper "Gloss on 'Gammon' in _The Waste Land_ , II, Line 166" (available on JSTOR), Sukhbir Singh points out that the word gammon has two meanings. Gammon can refer to a type of food, specifically the cured hind leg of a hog. But gammon is also a verb meaning "to make pretense" or to say deceitful things. On a very basic level, the use of the word gammon is a pun referring to both the dinner and the implied affair.
The pun goes a bit further. Gammon was thought to be an aphrodisiac, so the word gammon is yet another reference to sex. And Eliot refers to sexuality again by describing the gammon as "hot"; hot refers to sexual desire.
These two lines make a comparison sexual desire and hunger for food. Singh makes the argument that this comparison supports the theme in _The Waste Land_ of humanity's degeneration. If you want to learn more about that, I would encourage you to check out Singh's paper.