I think the in-lore answer has to do with the "bonds" Simon has with the rest of the harem, we've already seen that Simon and the harem can share memories
You might be thinking "but no one of the harem was involved in the Orcent x Kara scene, how can Simon use his magic to access that memory?"
There's actually lore answer to this. Orcent is also in some kind of 'bond' with Simon, almost similar to the bond he made to his harem.
Simon is an IK, and IK makes 'bond' by subjugating others. Its more of a thrall, but its a link between IK and their 'servant'. Simon just made the link to his harem and trusted minion in a more balanced relationship level, instead of master and servant relaptionship level like other IK.
When we first met Orcent on that burned farm on Stineford, remember that Simon uses his IK power to thrall Orcent into his power/domain so to speak.
(I think its because Orcent is heavily injured there, and by giving IK link to him we shared our power and sorta healed him. IIRC his speech intelligence is also increased a bit after the link)
I'd say this link/thrall/bond is the same thing Simon used to his harem when they have 'locked loyalty' to him.
And thats why Simon can access Orcent x Kara sexual memory.