Regarding the threesomes and other men, there is an MMF threesome, but it is 100% optional.
The MC talks to the girl about it, and they agree to it, meaning you have to specifically choose that path to see the scene.
If you don't want to see other men in sex scenes, you can simply ignore that option, finish her main story, and unlock all her regular sex scenes without any issue.
There is also a dream option in the game for a threesome, and again, it's completely optional.
So to be clear, even with the MMF options, there is no NTR involved in the game.