I'm not worried even remotely that he's cheating on me (and wouldn't read this as evidence he was), but I am jealous. I am also worried that his boundaries at work should be better and maybe I should say something.
Answer: The important thing is to present it not as an accusation, but instead as genuinely trying to understand something that struck you as odd. You want to know why it's bugging you, and how he sees it - including how he'd see it if a male colleague ate off your plate.
Spell out that you're not accusing anyone of anything, you're just chewing (sorry) on this. That leaves him room to say, "Huh, I never thought about it that way, I guess I am sending a bad message," or to make an argument that it's nothing - which will give you room to learn something from his response.
Comment: Is eating off someone's plate really a sexual thing? I'm not a fan of it in general, but often have noticed groups of women pick off each other's plates. If the situation is hetero (or if those women are lesbians) does this become more intimate? Maybe this woman is just a picker and has been eating lunch with the husband often enough that she doesn't ask and he's used to it.
Answer: Exactly. I see it not as a sexual thing so much as an intimate thing - which can be anything from harmless to marriage-threatening, depending on how he responds to his wife's honest expression of discomfort.
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