I don't know how to respond to questions about monogamy when I'm still closeted to my family. I'm in a relationship with one person at the moment but we're polyamorish, and my partner and I communicate very well. But my family have asked what I'd do if he cheated and tell me that I shouldn't trust him when other women stay at his place. I don't know how to convince them without explaining the situation that I can trust him and that my definition of cheating isn't sleeping with someone else?
It may be the case that they suspect something is up with you two, and are trying to pry it out of you with these weird questions. You may want to consider, if it’s safe and you and your partner are on the same page, just coming out to your family and explaining everything at once instead of trying to navigate these individual interrogations.
But, you totally don’t have to. This is the kind of situation where polite vagueness, feigned confusion and a quick change of subject is key. If they ask you what you’d do if he cheated, you can say something like “oh, I don’t think about that much - but we are shopping for a new kitchen table, want to see what we’re looking at?” or “that’s a bridge I’ll cross if we come to it. How are things at work?”
You could also get a bit more aggro and say “I find it pretty awkward to discuss issues of sexual fidelity in my relationship with my family, but trust me, I feel pretty confident in Charmoor’s and my relationship. Can we talk about something else?” or ask them point-blank “Why do you bring that up? Do you have a question or concern about me?”
Ultimately, though, the issue of “I don’t know how to convince them” is a common one in families. There are no magic words we can say to convince people to understand something, and with families that is so much stickier. We all have an aunt who’s convinced her dog has a gender identity, or a cousin who’s in an MLM, or a grandfather who can’t stop asking us when we’re going to law school, and sometimes we just have to let people be out there in the world, unconvinced of entirely reasonable things despite our best efforts.