Yea - living together requires a lot of forethought and space. Benn there- done that - probably will again but wiser now.
What we've learned - and kind of formed requirements around....
1> Everyone needs to have their own private space regardless of how close you are. This is not a bad idea even for two people ! That private space is pretty much 'off limits' to anyone else unless it's an emergency or via invitation.
2> If anyway possible, create equivalent 'public' space, but in order for even this to work, you have to communicate solidly about what expectations are to be met - be it cleanliness, activities etc.
3> Private baths are also almost a must have. It's amazing how silly little things can annoy someone else and cause problems. i.e. a stray hair left behind etc
4> Sex needs to have space for that spontaneity you speak of and everyone has to understand that. A little common respect and decency goes a long way. If two have it going already, any third should expect an invitation to join and NOT take it personally if that invitation doesn't come. As you've discovered, those special moments just show up unannounced and everyone should have no desire to change them ! It's different when you all start together but coming late to the party needs to have a penalty
Out of time.............maybe more later.