I'd just toss it back to them -- why not?
But, if you want something a little different, perhaps "Because it suits my personality and desires, and has been a good fit for me and my partners. I know it's not for everyone, but I appreciate the honesty and freedom."
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.