Well relationships like that really are rare and unlikely, M&J is unique in this game and it probably will stay that way. I know it's a game, but the writing still needs to be somewhat plausible. Usually when you care about someone, you don't want to share them. And since MC's relationships on his paths will only keep developing further, I really don't see anything like this happening with the main girls except in special renders. You just couldn't implement it in the story without it looking too ridiculous, this is mostly just people fantasizing.