Monogamous person here. I would not be offended if someone asked me if I was, or if someone else is. I think it's kinda hot. It's not something I'd want to be in long-term, but for a night? Shit, why not? That said, I've been married almost 20 years, and my wife and I aren't looking for a third or a couple to play with. Maybe when we were both 20 years younger. Not now. That's a younger person's game (to my mind anyway). But offended? No way. You do you, boo.
You can also ask me if I'm gay or trans. I'm neither, but I won't mind being asked as long as it's respectfully. And I'll answer in kind. I have been hit on by men. Not often, but it's happened a few times. Always flattering, as long as they take no for an answer. They always have. I mean, it's kinda dangerous for them if they don't. Not from me. I just mean in general. And violence against LGBTQ+ has always been a factor, unfortunately. Still, I'm flattered they think I'm worth their time. But I don't swing that way. I thank them for the compliment and wish them well. I think it's the best you can do in the situation. And while best is the enemy of good, well, I try to be better.