Okay, I think I can get what you mean here. And if I may I'd like to respond with an anecdote.
WARNING: ANECDOTE FOLLOWS (the reason for saying this will become clear in a moment)
My in-family partner, whom I'll call KL (to distinguish her from my out-family partners K, L and T - KL is the mother of my son, and may therefore be assumed to be a "primary" (although I despise that term)) currently maintains a relationship with one of my very good friends TC. Neither TC nor I have any homosexual attraction to one another. However, KL has recently decided to become submissive (for reasons I completely understand) and I cannot be dominant in the way she wants. TC can. So KL now has a sexual relationship with TC, but not with me. However, she is still the mother of my son, and my feelings for her will never change. TC and I are, as I said, very good friends, and that doesn't look like changing either.
So what you suggest doesn't really apply in this specific case. But it certainly may in others.
Please remember that I am speaking about my own experiences only, and what I say here should NOT be taken as a general comment (the reason for saying this will become clear in a moment).
I don't know about that. It certainly isn't true in my case.