I don't think I understand your example, but that may come from having had more than my share of polyamorous friends.

By default, I would take your request at face value and have no trouble saying, "No, we're monogamous, but I can't very well blame you for wanting to!"

very well, but what percentage of the population do you think would consider that rude.

Of course, the nut of the question is whether its ever possible to be rude with a question. If it's possible to be rude with a statement, I don't really see the difference between questions and statements, at the higest level, though

Thinking about it, it seems questions generally allow for some flexibility of interpretation reducing potential cause for offense, where statements allow far less latitude. The problem is that some questions require foundational understandings that may be offensive, or can really only be understood with malintent. "When did you stop beating your wife?" comes to mind. The question doesn't even allow for the possibility that you don't beat your wife.

Similarly, "Can I sleep with your wife" implies that the asker thinks it's a possibility worth inquiring about. So I basically agree with you on a practical level -- most people would not respond well to the underlying assumption. The question is a veiled statement ("I think you might be open to letting me sleep with your wife, so here I am asking"). In that sense, the question and the statement aren't very different, as you noted.

Unless I have other context to assume ill will, I've fought for years to train myself not to assume it. Whenever I can, I try to interpret something in a light where the other person means no offense.

Certainly sometimes I'm wrong. Most people are assholes sometimes (myself included), and some people are assholes most of the time. But I've found my conversations go best when I try not to assume assholery about someone for as long as I possibly can.

In this case, assuming someone's worldview doesn't take monogamy for granted, and that they've also noticed how sexy my partner is, is how I avoid being offended, even if they frame the request in a somewhat crass way.

Even if you someone was polyamorous, the question is still offensive as it implies ownership and authority.

I think the "with you" is implied: "Is it OK with you..."

I can't imagine a context in which someone would bother asking me for permission to rape my partner.