Ok I'm gonna jump in here because I think I know you a little and I kind of feel like you're getting picked on a bit.
No one is picking on Mono here. We're trying to understand and help answer these questions. He got defensive for no reason. Quite frankly, the whole situation and questioning are confusing. And I am new to poly, mostly familiar with mono relationships, so it's not just poly peeps who find his mono viewpoint hard to understand. I'm from New York, too, so I guess I feel a sense of relief when someone says something directly instead of using metaphors. Why pussyfoot around issues involving pussy? heh-heh.
Mono, The confusing part to me is that I understood RP has not had sex with Leo, thus respected your boundaries. But the night he came over and stayed in her bedroom started off a whole slew of discomfort for you, Mono. Yet you say you're not jealous. Yet, RP apologized in other thread for breaking your trust. What did she do? That is still not clear. What does she want to do? She has said she will not move forward with Leo and still there is a shit storm to deal with. What does stepping back mean for you, exactly? You sometimes wax poetic and are somewhat vague in describing certain aspects of your situation. We're all just trying to understand what the hell you're talking about. No one is picking on you or posing questions as a way to bitch at you or poke fun.
It does look as if, from the outside, that your boundaries (which really come across as rules to me) have essentially placed RP in a mono relationship with you. You have managed to exert some powerful control over her. I know she has said that the sex with you is more satisfying than it is with others, or at least, being with you involves lots of sex. She's not sexual very often with her husband or other partners who were (I believe) in her life before you came on the scene, so it's like she's living a pretty much mono lifestyle in deference to you, despite the fact that a poly tribe is something she has said she strives for.
I don't think your stepping back is an unreasonable shift to make in order for you to handle the addition of another partner in her life (as long as you won't be pouting about it). And while it's ridiculous to think that adding someone new to the mix wouldn't demand changes all around the tribe, you seem unwilling to look further into why it would bother you so much. It's like you've dug your heels in and said, "this is just the way I am" without wanting any suggestions that there could be jealousy or other issues at play here. But, okay, let's say it is just the way you are and you do step back and RP takes on Leo as a full-on sexual partner and lover, and maybe other people, too, for that matter. How will you handle what may happen if lovemaking with Leo or another eclipses lovemaking with you, just as lovemaking with you eclipsed lovemaking with PN. Are you prepared for that? Maybe you can handle her relationships with PN and Derby because on some level you feel you've got the upper hand and are superior, or preferred, in your lovemaking, but Leo is an unknown. He throws doubt into the mix. She could wind up preferring his lovemaking over everyone else's. Hmm.
Even though you are mono and feel the need for connection to be sexual with her, I fail to understand how other, additional relationships RP has would be more of a threat to your connection than the ones she currently has besides you. It just does not make sense. If you allow some and not others, it's not quite poly, not quite mono, but you remain in control.
Perhaps this whole issue is rooted in your need for some sense of control in the situation. I'm not using the word "control" negatively. Human beings do often find comfort in some sense of order in life, and feeling like we have a handle on things. Perhaps, however, your need to be okay with the relationships RP has could be better handled by your own personal work on yourself rather than imposing limitations on whom and how RP loves and makes love to. She is poly, after all.