Alice has a thing for Bob. Maybe they've been friends for a while and she's ready for a Relationship Upgrade, or perhaps they just met and it's Love At First Sight, but regardless, she's perfectly smitten. But there's one problem: Bob is gay, and he's already in love with Charlie. To meet the requirements of this trope the relationship must be stated as hopeless or impossible in-universe (by a character or through Word of God) at some point in time. This means that Incompatible Orientation doesn't have to be a permanent state of being, and attempts may be made within the plot to 'fix' it.