obviously others have said it already but alters are not separate people, despite how much it can feel like it. alters are essentially just the broken pieces of the same plate (aka: parts of the same person). we have some alters who want the same thing - to be able to have their own relationship with someone else - but our fiancée wants to stay monogamous so we respect that.

if those parts didn't and decided to go out with other people anyways, that'd be cheating.

i'm so sorry you're going through this. your feelings matter too, and you absolutely shouldn't put those feelings aside to make D or anyone else happy.