One possible lead-in for this quote from The Great Gatsby could be:
"During a conversation about the marriages of Tom and Myrtle, one of the characters expresses their thoughts on the matter."
Alternatively, you could provide some context for the quote by describing the situation in which it is spoken, such as:
"As Tom and Myrtle's infidelities are revealed, one of the characters suggests that they should leave their respective spouses and marry each other instead."
You could also set the scene by describing the setting or the mood of the conversation, such as:
"As the partygoers at Gatsby's mansion become increasingly intoxicated and candid, one of them offers their unconventional solution for Tom and Myrtle's unhappy marriages."