Hi and welcome
You're in the right place. Monovcphg is a mono but I don't think you could call him a secondary. My partner has "secondary" and we have decided that "secondary" is a label that does not do their relationship justice. We now refer to the points on our V as "partner" (because he and I have a committed life together) and "Significant Other".
In our V both SO and I are currently mono so I have some understanding of where you're coming from. You're doing very well I think; being a mono SO is just as difficult as being a mono partner I'm sure, but obviously for different reasons.
Your life will probably evolve in one of two ways, you'll either become more involved with your lover's family (like monovcphg), have you talked about that? Or you'll find another relationship that you will integrate or not depending on its nature.
Don't worry so much about your effect on your SO and his wife, it sounds like they have a good handle on things and your rights and feelings are important and valid. You have enough to deal with, let the guilt go.