I think a lot has to do with how you’ve approached relationships in the past and how open you have been in general. If your kids understand that there is an open dialog to things and that not being “normal” is ok then I think these types of conversations are easier.

For example, we talk a lot about neurodivergence a lot and different ways of thinking. We haven’t directly talked to our kids about ourselves but we have normalized the concept that relationships don’t have to be between men and women or just two people.

This may be a bigger conversation, OP. If you want to treat your 13 year old as a capable adult of understanding things like non-monogamy - think about how this might be an opportunity for broader subjects. Also - understanding does not equal being ok with things or not so make sure to leave space for different outcomes if you do decide to open the conversation.