The title does a lot of work here. "In/mates" splits into two ideas: people who share an intimate life, and people confined together, like prisoners. The poem seems to hold both meanings at once. A husband and wife sit down to eat, but the meal is a heart, and all it offers is "gruel and need." Gruel is thin, bare food, the kind served in institutions, so the marriage feels both tender and cramped, bound by hunger more than comfort.
The second stanza turns toward mystery. "Who knows what kind of night / began as a cell" could point to a single biological cell, the start of a life, or to a cell as a small enclosed room. Either way, something vast grows from something tiny and closed in. The speaker sounds wary and a little awed, unsure what darkness or beginning they share. The mood is spare, intimate, and unsettled: love as closeness, but also as need and confinement.