Arato is objectively a lot better. That's the point. That's why she's with him and she admits it (and confirms it with her flashback) she needed someone with enough sexual appetite to keep with her, and she wanted someone on top. Arato fills all her boxes (young, rich, very sexually active and to boost he's a dumbass she can deal with relative ease) with the only caveat that he's a bastard and is emotionally distant. She knows this and lampshades it.
Kendo has a mega cock and can give god-tier sex, but that alone isn't enough for Shizuka.
Its also why Kendo knows that it would be difficult to get her. Man may lack a lot of things, but he's experienced and he only needed glance to realize this.
I wouldn't say its impossible for Kendo x Shizuka scenes to happen, but its kinda obvious that despite her horny overtaking her at times, Shizuka is actually able to see things objectively unlike Mary.
Also I really want to see Arato getting cucked to hell, so its fine either way.