If the only romancing character is Alice, I don't know what the reference to creating a harem for the MC is about. If all the MC can do is participate in scenes where other guys are involved, that's not a harem.
The only reason I see for the game's plot to need voyeur scenes is to want to make a voyeur game. If it is that what you want is fine, but then it is best to make it clear and not imply anything related to a harem. In my case, I don't want to see any scenes where the MC is not involved, and I don't want to see any scenes where a guy other than the MC is involved (i.e. the only scenes I want to see include the MC with one or more LIs without lesbian content involved, and I prefer that all the ones I don't like doesn't happen)
If you don't want to make a voyeuristic game, then I don't see what those scenes bring to the game: If you really want to make a harem game, then those scenes are not a great idea, as the base players won't like them. If the idea is to make a romantic game between Alice and the MC, almost no player who likes to romance a single LI is going to want to see voyeur scenes, so they're not a very satisfying inclusion in the game either.
The argument of making the game more realistic and inmersive doesn't convince me at all, I prefer a realistic and inmersive game where the only sex scenes are between the LIs and the MC (or between girls, LIs and non-LIs, and the MC, although I'm not interested in sex with non-LIs). I don't know why it has to be more realistic to watch other guys fucked, and I certainly don't find it immersive at all, unless unpleasant immersion is the goal.