About Melody and Fi hooking up later on, if you don't want that to happen, you'd have to turn Fi down as Melody pretty early in the story, I think it was when Arnold told Melody to leave for a few days because he needed time to think, so Melody crashed at Fi's for a few days. I think it was there when Fi kisses Melody if I recall that right. And as Melody you can choose how to respond to that, is Melody okay with it (and possibly be into girls as well as guys) or not.
Also, regarding the possible threesome, I think it was when Fi and Melody went to sunbathe or something, I think Fi was asking if she could be taught by MC as well, and I think she was dropping innuendos as well, and again as Melody you can choose if you're fine with that, or would you rather keep MC to yourself.
I might recall wrong, but I think those were key starting points for possible FF and MFF content between Melody, Fi and MC.
By the way, technically a girl remains a virgin until she's been penetrated by a dick even if she broke her hymen using toys, it's a technicality, and, not all girls are even born with a hymen.
I mean, if penetrating yourself with a dildo makes you not-virgin, then in same sense a dude using a pocket pussy should make him a not-virgin, too, no?