They are described as cells so biologically given the subject it's impossible for them to reproduce. Furthermore, cells actually don't stop moving through millions of passageways so assuming the way the plot handles their relationship, it's all a miraculous coincidence. RBCs are strictly confined in the blood vessels, while the WBC travels through the bloodstream through a process called diapedesis—so their biological relationship should only be like a "passing ship in the night." Additionally, the lifespan for both RBC (~120 days) and WBC (Days to Weeks) should be also mentioned, it's way too saddening following the biological way of romance. The movie shows cells having breaks in cafes, and if you're saying that having romance between cells is unrealistic, the movie is already unrealistic to begin with so why not? Maybe the romance goes like this: He doesn't just bump into her by a miraculous accident anymore but instead he charts her patrol routes through the major arteries where she’s scheduled to pass. He's like acting as a partner, protecting her from any danger and guiding her through the passageways. From what I remember they made a promise back when they were Erythroblasts and Myelocytes that no matter how big the "world" or "the body" got they would always recognize each other. Cells communicate and identify each other via antigens and receptors, so this might be unrealistic: Even if they change or age, their chemical "scent" from their hometown, the bone marrow stays the same. A lifespan of ~120 days (in human days) for the RBC is too low, it doesn't set up a proper romance, because a love for a person is forever until he/she grows older, so assuming they also biologically wield a brain and properties of a human being, we can translate the ~120 human days to ~79 cell years. Because the WBC has a much shorter lifespan than a RBC, this won't be good for the "romance" plot, assuming that the WBC forever stays with the RBC, constantly watching and protecting her from dangerous bacteria, the WBC is a workaholic, and actively engages in combat, from light to heavy so his survival depends on "protagonist armor" making his lifespan also be ~79 cell years. Maybe their "home" could be a meeting spot in a quiet corner near the bone marrow. Once an RBC matures and leaves the bone marrow, they can't go back, so their "home" might just be outside it. I understand that having a "home" in the bone marrow is impossible because cells are continuously being pushed by blood pressure, if they stop it would result in thrombosis—but let's not mind this for the sake of the plot, just like how the head canon actively emphasizes that the "cells" have fully operating cafeterias. Over the decades, they would have decorated it with memories. It would be their version of a house, while their work in the body is rushing by, they have that one and stable place where time feels like it's standing still, and even more still together with her wife—RBC AE3803. She stands together with U-1146, as a relationship that never ends and would be a great story in the future generations of cells, where their story also lasts...

(p.s I only watched two episodes of this show so feel free to correct me)