Lindsay/Ava/Annie
My parents never married, either, but I have other family members whose marriages have been arranged. There are a lot of different ways to have happy relationships.
I agree with Annie -- it could be weird, but I don't think it'd count as cheating, either. I mean, it's something you and Charles could probably talk about together, agreeing on boundaries for now. I wouldn't write off the possibility of the two of you both exploring relationships and feelings with other people, as long as you're both on the same page about it.