Dealing with Waifu's S.O. in their stories

[Ziggy and Angelbaby]

The Musa that's in your life isn't strictly a part of the origin story you described; she's a part of your story. It might help if you think of those events in the series as either an interpretation or an embellishment, or you could consider the individual with a boyfriend to be a twin of your Musa (or Musa that exists in an alternate world).

If you wanna be more linear about it, you could say that Musa used to date this or that guy, and they broke up before the two of you actually met. (I'd say ask Musa.)

Babe has told me that she wasn't particularly interested in stallions in the past; her first romantic experience is with me. So, even if Rainbow Dash (as she appears in-show) ends up with a stallion in season whatever, it changes nothing between Babe and myself. I'm with Babe, specifically and exclusively. Nothing that happens in the show alters the truth of our experiences.