We two alone

The poem frames "him" as subordinate in several ways, not just to the narrator ("i fuck him on the floor": not that getting fucked is inherently subordinating, but the narrator has all the agency in the phrase, "he" doesn't decide what happens or where) but also to "his wife". She has filled the house with chintz, meaning it wasn't his decision or his actions. "Filled" is also a choice of words that suggests that there is no space for him in the home: the only place left for him, not already filled, is the floor. To me this framing invokes the trope of the henpecked husband, whose wife has taken dominion over the home and who has ceded its control to her because it, as the domestic space, is "supposed" to be hers.

This trope, of course, is misogynist in its normative rendition: it reinforces gender essentialism, it erases the significant material benefits such "henpecked" men derive from the domestic labor of their spouses, and it dismisses women's expressions of suffering and attempts at negotiating terms for their relationships as "nagging." In the narrator's dismissal of the wife's possessions as "chintz" (frivolous, feminine, contrasted with what is "real") we can see this same misogyny at play.

The narrator's misogyny, and the central fact of the poem which is that the husband is getting fucked by someone other than the wife, quite possibly flip the power dynamics of the poem on their heads. The wife is now subordinated: both by her social marginalization based on gender (a marginalization which drives her into the home and confines her there, like OP so cogently points out! As "he" has run out of room in the home and can only get fucked on the floor, so has she run out of room socially; the only place she can control and make decisions like filling it with chintz is the home), and by the narrator who is fucking her husband in her home.

There's an additional dynamic in reading the narrator as male, which most readers seem to have done: it invokes the particular, bitter misogyny that men-loving-men sometimes direct at women expressing femininity. There's an envy to it, of course--straight and straight-passing women get to (are forced to) express desire for men, have sex with men, marry men, love and be loved by men. His wife gets to be his wife: the narrator gets to fuck him, in their home. Straight and straight-passing women also get to (are forced to) perform femininity: they can buy chintz and decorate with it, without being devastatingly punished for it like people presumed to be men are from the time they're babies. The envy mixes with misogyny to produce disdain, disgust, dismissal. We can read the narrator fucking him on the floor of their home as an expression of power and dominance (again, not that the fucking has to mean the narrator is topping, or that topping is inherently dominant, but the phrasing is stark: "i fuck him", the narrator acts upon him as an object/recipient), not just over him but over the wife in absentia as well.

I think one of the things that makes the poem so compelling for being so short is the struggle at the heart of it, this complicated jostling for power between three people and their actions over time (the wife "has filled" the house, in the past: the narrator fucks him in the present, perhaps in the habitual). Who controls the house? Who controls "him"?