What I'm gleaning from this exchange (and I feel so fortunate to be able to hear your perspectives) is that a secondary relationship has the potential to make either person feel less valued. As the married person, I have been feeling less valued because of C's intention to find a full time partner who will almost certainly cause our relationship to change to a casual friendship with very little contact. As secondaries, I hear you saying you can feel less valued both because of the imbalance of the relationship (getting less time than the primary) and because of a fear that the married partners might call an end to the secondary relationship at any time.
I don't believe in "veto power" and would never allow another person to dictate who I may and may not spend time with, or love, but I do know that my relationship with C has had to accommodate several changes of shape (amount of intimacy, amount of time together, kinds of time together) due to my husband's feelings. I've always felt this was inherently "unfair" to C, just as I have felt it was "unfair" to my husband for me to start living polyamorously after many years of monogamy, but I've tried (and sometimes failed) to let each of them know how much I value them.
I'm also concluding that the mono secondary relationship is inevitably going to end in a lot of pain for both parties, unless it can take a very large place in the married partner's life, as LovingRadiance has achieved with her live-in boyfriend. Since my husband will only befriend C if we aren't intimate, it means having to always divide my time between them, which severely limits how much time I have with C. Since sex isn't an option right now, it means C is never going to feel totally accepted. I see the dead end in this now.
When C gets back from camping, I want to walk to him about ways I can still feel valued in spite of the inevitable loss of this relationship, and ways I can help him feel valued in spite of his "secondary" status in my life.
Married to a monogamous man 15 yrs, mother of 2, dating C 3 yrs, and in a romantic friendship with L more than 20 yrs