I am fairly new to poly, and I am bisexual. So is my primary, who is a woman. We allow each other leeway to seek out lovers of any gender (M, F, trans, queer). It would never occur to me to ask her to be present w me and one of my lovers (all of whom presently are male), to look but not touch. I mean we are open to a 3way should it happen, but it's not a requirement.
Why does she "need" your presence when she's sexing someone else?
Wouldnt it be fun enough for her, just with this hypothetical lover, one on one?
On the surface, it looks like she loves you so much, she wants you there to enhance it, or it wouldnt be fun for her. But, otoh, I am getting a D/s vibe from that. Do you know much abt BDSM? It seems like it's a turn on to her to limit your participation to watching only. Could be a form of sexual torture.
Do you feel that power dynamic in your relationship presently at all?
And the "only after marriage" thing seems a bit unusual, fetishistic. Almost ritualistic.