In theory there's nothing wrong with this. Just like you can decide which girl you wanna romance the most, you can choose if you want to see the girls fuck somebody else. It's just a game, you're not actually getting cucked.
The issue is that, in reality, it's almost unheard of that a dev can successfully make a game with 2 very different story lines.
It's too much work, one side always gets more than the other and you end up with 2 factions of fans that despise each other and half of the game
Harem games that include NTR content are basically just cuckold games:
In the romance route (slow-burn type), the heroine pretends to be a pure and holy nun (the protagonist can't creampie her, can't force her, can't fuck her hard—basically, this isn't allowed, that isn't allowed).
In the NTR route, the heroine switches on her slut mode (allowing other men to creampie her, abuse her, humiliate her). It's always this same clichéd, cookie-cutter script.
Who would like this pretend-pure-nun romance character? Once you've played the NTR route, and then go back to the harem route, you'll find that woman pretending to be a pure nun absolutely disgusting.
Optional NTR harem games? There was never any choice to begin with—once NTR content is included, it's decided: your romance character will become even more slutty and submissive in the NTR route.
