I'd say it's more about it being a porn game where the the MC is an extension of the player (we make decisions on the MC's behalf), whereas in regular porn you're just watching characters. This can give sharing scenes an unpleasant, cuckold like experience for players (at least for some). I'd also say that in regular porn stories it's more about characters who are promiscuous; not a lot of emphasis is placed on relationships and infidelity, and in cases where there is a lot of emphasis there, it is often categorised as a 'cuckold' porno -- and this is unappealing to many.
I do agree that thematically the LI's sleeping with other men can make sense, as was the case in Pretty Woman, but most people who play this game do so for sexual gratification -- the same can't be said for viewers of Pretty Woman. This does speak to a tension I see within the adult visual novel space: where creators have a hard time balancing a utilitiarian commitment to 'broadly appealing fap material' and an artistic desire to create a good story.
