You're cherry picking and in a way I would agree that in a vacuum none of the negative answer would suffice to push her away, but you're not considering that Sue is not specifically on a love path with Randall. All paths have in common that she want to become a Bimbo and be free sexually. What that means differ from path to path and in the path in which she can have sex with Theo that night she's been sexually frustrated by her son fucking Scarlet every single night for hours. She has also been insulted, demeaned and reduce to a sex toy by Randall who has still not forgiven her (Again, I'm not taking any single answer alone but compounding all negative answer, The system itself just check if you have 1 or more of Romance and Loyalty). Randall is doing it on purpose, So what does a sexually liberated woman (or at least one who working hard to be) do when she's been horny as hell for days and clearly see the person who she love, but is not ready to commit and act on it, fuck another girl in front of her? Well there's a gentle and kind boy sleeping in the next room over. A boy she know find her attractive, who won't treat her like Randall is doing right now and will neither deny her or be jealous if she goes to someone else after. You might disagree, but the reasoning is sound here, She doesn't act out of nowhere here.
As for you mods comment, you don't need any, it's in the options.
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Mikey Mike
Lucia Loyalty doesn't matter here unlike Sue, she only go see Theo if she has 0 Romance point or less.
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