As I understood, the Mr. Green character was described in the plot as being blackmailed because of his homosexuality. I watched this half-asleep & didn't follow much of it- I just want to understand why the line was used for the closing freeze frame.

He says early on at the party, "I work for the State Department.. and.. I'm a homosexual". So is the gag that he is a homosexual FBI agent in real life (and prior to the night of the party)? He was being blackmailed because of his homosexuality, he is now still homosexual. I guess the joke is either a) he is continuing the prior illusion that he is straight to fit in/keep his job despite everyone knowing the truth b) he is making a sarcastic joke because he is gay & everyone knows it. Or a combination of the two. Or is the gag that he is heterosexual, an FBI agent, and is actual going home to sleep with his wife? (After pretending to be gay for the night which the movie takes place during).

Green says he was a plant.. ..but how would he ensure he was invited, if he was gay IRL. I could see him going to his superiors and coming clean/explaining the blackmail situation, but it seems like a realllll stretch that he'd then know he was invited to a party that would involve murder etc. ..if he was an ordinary heterosexual FBI agent that was chosen for the mission, then wouldn't Mr. Body etc. recognize that his physical appearance was the incorrect person?

A lot of the plot was bad, and I understand that. The writing isn't great, there are plenty of plot holes. It was just such a weird way to end a movie.. the closing quote that was more intrigue than humor, + the freeze frame, + the music choice.

Maybe the audience is supposed to assume that: -Green is a gay FBI agent. He is blackmailed, and invited to the party. -At some point during the night, he calls J. Edgar Hoover, and explains that he is gay/has been invited to a party/murders have happened. -Hoover then sends more FBI agents to the house to arrest either Body or Body + multiple guests. -Green is now outed publicly. He says, "I'm gonna go home and sleep with my wife!", in an effort to humorously smooth things over/make it clear that he'll continue portraying himself as heterosexual? But then he wouldn't be described as a plant.... that phrasing makes me assume it was an operation planned in advance. Also, why would Body knowingly blackmail an FBI agent, that seems kind of like a foolish thing to do.

Another possibility is that the FBI had a long term investigation into Body. They used Green to fake (or not) homosexuality deliberately to snare Body. Time passed, and Green was eventually invited to the party.

On a side note, I think I've heard the quote used in other media. It made me think of the Rick & Morty episode, "Jerry's Perfect Simulation", where the character says enthusiastically, "I'm going to make love to my wife!". But that is not nearly identical phrasing. Is this a commonly said thing in movies?

The answer is probably, "the writers didn't care or think about it for more than the minute or two which it took to think of the joke. They lazily left so many open possibilities that the joke is that Green is either a gay man joking about sleeping with his imaginary wife, or a straight FBI agent joking that he's now done playing the part of a gay man". If that's the case, I think it's a terrible move by the director & editor to use that as a freeze frame ending.