"What's that?" I said.
"This Chris Huhne, did you read about the woman he's dating now?"
I had, but I shook my head. "She's been married twice before, to women."
I blinked. "And?" I said, failing entirely to see what she was driving at, other than being slightly scornful.
"So, she's bisexual. Don't you think he's worried she might leave him for another woman?"
I blinked some more. "Um, probably not any more than he would be that she might leave him for another man?"
She stared at me. "I just don't understand that at all."
I shrugged despairingly, and changed the subject.
In another reality, a braver, bolder me made the following speech so compellingly and humourously that she would have understood. "Sexual orientation and faithfulness are not dependent characteristics. Being bisexual doesn't make a person more or less likely to be unfaithful in a monogamous relationship. I'd be willing to bet there is at least one heterosexual person on this train who is in a monogamous relationships who has cheated on their partner. I'd also be willing to bet there is a bisexual person who has never cheated on anyone at all, and a heterosexual polyamorous person who has two husbands, and an aromantic asexual person who has never slept with anyone (and, incidentally, is perfectly content), and a gay person who has only ever had one partner and has been with them for fifty years. I'd even be willing to bet there is a person on this train who enjoys sex the most while wearing a Batgirl costume and is impatiently waiting to get home to her - or his - Stormtrooper. That isn't me, by the way, so you can't win the bet by guessing that."
I'm not that braver, bolder me. But I would like to be.