That's the thing. Like every fetish, there are probably people who aren't big fans of group sex.
If sex scenes are optional and at the player's choice, it would make sense that threesomes and group sex would be as well. If I want to romance all the girls and go full harem, I don't see it as "fair" that I'm forced to have threesomes with the girls just because of this. There are girls that I wouldn't mind having threesomes with and there are others that I would like their scenes to be just one on one with the MC.
That's why I think having the option to agree to have them and refuse them would be fine. For site rules and out of respect, I will not mention the name of other games. But I will give an example. There is a game that uses the same graphics engine as your game that deals with hypnosis. *wink wink*.
Well, in that game threesomes are optional and there is no punishment if you refuse them. I mean, you don't miss scenes since, if you reject threesomes, you have individual scenes with the girls involved. So the one-on-one and threesome scenes are 1:1. You receive the same amount of content regardless of what you choose. I know this is more work, but it could be an option. So, it would be nice if group sex was optional no matter how many girls you want to romance.