The poem uses antithesis to underline differences—especially racial differences—in the community it portrays.
For example, the speaker describes one of the two young men as "white" and "golden" like the "day," the other as "black" and "sable" like the "night." (Sable is a kind of luxurious, dark fur as well as a synonym for black.) The speaker also contrasts "the dark folk" who "stare" at the young men from surrounding windows with "the fair folk" who "talk" about them as bystanders.
These details suggest a neighborhood whose residents are predominantly Black—a reflection of Cullen's highly segregated America—but whose streets contain people of different races going about their business (or spying on other people's business). That the Black observers simply "stare" while the white observers "talk" may suggest that the latter are more vocal, more proactive, about enforcing racial boundaries. This contrast appears again in the phrase "look and word" in line 9.
In a way, the poem also sets up an antithesis between difference and similarity! For example, the two young men are of different races—but they're both young men. They're "Locked arm in arm" and "walking in unison": phrases that connote similarity and harmony. (If they're lovers, however, they face discrimination due to another socially defined distinction: straight vs. gay.)
Likewise, the "dark folk" and "fair folk" have contrasting skin tones and react somewhat differently to the young couple—yet they seem united in their suspicion of that couple. That is, they both seem to uphold, or at least act in accordance with, a social code that treats race as a fundamental source of division.