Jill's character - Incredibly naive or just dumb
Here she (Jill) is, seeing her boyfriend (Luke) and his coworker (Kate) flirting with each other openly at the company party in front of everyone, she goes on a double-date to a resort house and sees Kate and Luke flirting, touching, hanging out late at night, Luke hanging out late with Kate at bars, Luke inviting Kate for dinner, Luke not wanting to talk about marriage, etc, and she (Jill) clearly doesn't even suspect anything.
Of course a probable rebuttal is because she's worried about an innocent kiss she had on the hike with Chris, but that still doesn't offer full explanation, because several of the above events happened even before that beach house trip...
She wouldn't have felt so guilty about an innocent kiss if she expected them to be attracted to one another. More telling, she wasn't looking for an omen or admission from Luke when she confessed to a kiss, which makes me believe she did not suspect anything.
Jill's characted must be incredibly naive, or just dumb.
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When logic and science aren't on your side, you always lose.